九州大学 研究者情報
論文一覧
清水 洋(しみずひろし) データ更新日:2021.04.05

学術研究員 /  理学研究院 附属地震火山観測研究センター 実践的火山専門教育拠点(特定プロジェクト教員)


原著論文
1. Keita Chiba, Hiroshi Shimizu, Spatial and temporal distributions of b-value in and around Shinmoe-dake, Kirishima volcano, Japan, earth, planets and space, 10.1186/s40623-018-0892-7, 70, 1, 2018.12, [URL], Kirishima Shinmoe-dake, located in southern Kyushu, Japan, erupted in October 2017 after 6 years of quiescence. Analysis of volcanic gas indicates that magma upwelling from depth affected this eruption. Shinmoe-dake’s recent eruptive history also includes phreatic eruptions in 2008 and 2010 and a magmatic eruption in 2011. We examined spatial and temporal variations in b-values to investigate the magma plumbing system in and around Shinmoe-dake and its possible effects on unrest. A region with relatively high b-values (b = 1.5) is located at depths of − 1.0 to 2.0 km below sea level beneath the summit. It is likely that this anomalous region was generated by crustal heterogeneity, as it colocates to pressure sources. We investigated temporal variations in b-values from January 2007 to October 2017 in two regions: beneath the summit and 7.0–8.0 km northwest of the summit; the latter corresponds to the region above the locations of pressure sources related to the 2011 and 2017 eruptions. An increase in the b-value (b = 1.4) beneath the summit was observed beginning in early 2009, followed by a subsequent decrease (b = 0.9) immediately before and during the 2011 eruption. Similar temporal changes in the b-value were also observed beneath the summit before the 2017 eruption. From these results, we can infer that the increase and subsequent decrease in b-value express the activation of small cracks, due to the generation of hydrothermal fluids, and the development of cracks, which produces higher magnitude earthquakes, respectively. Meanwhile, a decrease in b-value (b = 0.6) was observed in the region northwest of the summit during the 2011 and 2017 eruptions. Thus, it is possible that the decreases in b-value in this region result from the activation of small cracks and the development of these crack systems and/or a change in the stress field near the magma chamber, which produce higher magnitude earthquakes.[Figure not available: see fulltext.]..
2. Satoshi Matsumoto, Yusuke Yamashita, Manami Nakamoto, Masahiro Miyazaki, Shinichi Sakai, Yoshihisa Iio, Hiroshi Shimizu, Kazuhiko Goto, Tomomi Okada, Mako Ohzono, Toshiko Terakawa, Masahiro Kosuga, Masayuki Yoshimi, Youichi Asano, Prestate of Stress and Fault Behavior During the 2016 Kumamoto Earthquake (M7.3), Geophysical Research Letters, 10.1002/2017GL075725, 45, 2, 637-645, 2018.01, [URL], Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex coseismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here we show the three-dimensional prestress field related to the 2016 Kumamoto earthquake. The estimated stress field reveals a spatially variable state of stress that forced the fault to slip in a direction predicted by the “Wallace and Bott Hypothesis.” The stress field also exposes the pre-condition of pore fluid pressure on the fault. Large coseismic slip occurred in the low-pressure part of the fault. However, areas with highly pressured fluid also showed large displacement, indicating that the seismic moment of the earthquake was magnified by fluid pressure. These prerupture data could contribute to improved seismic hazard evaluations..
3. S. Katakami, Y. Yamashita, H. Yakihara, Hiroshi Shimizu, Y. Ito, K. Ohta, Tidal Response in Shallow Tectonic Tremors, Geophysical Research Letters, 10.1002/2017GL074060, 44, 19, 9699-9706, 2017.10, [URL], Various types of slow earthquakes (e.g., tectonic tremors and slow slip events) have been reported in tectonic zones, especially in the subduction zone. The tidal response of a tremor is considered to be strongly related to the weak friction state of the plate interface, and many studies have reported observational evidence of such correlation between tides and deep tremor activity. Here we used the modified frequency scanning method at a single station to detect micro tectonic tremors that have not been previously reported in southern Kyushu. In the early stage of the tremor activity, tremors are mostly modulated by slow slip events. In contrast, we found a seismic response to ocean tides during the later stage in the shallower part of the subduction zone. This might indicate that the tremors are triggered by tidal changes caused by fault weakening due to slow slip events as same as deeper condition..
4. Azusa Shito, Satoshi Matsumoto, Hiroshi Shimizu, Takahiro Ohkura, Hiroaki Takahashi, Shinichi Sakai, Tomomi Okada, Hiroki Miyamachi, Masahiro Kosuga, Yuta Maeda, Masayuki Yoshimi, Youichi Asano, Makoto Okubo, Seismic velocity structure in the source region of the 2016 Kumamoto earthquake sequence, Japan, Geophysical Research Letters, 10.1002/2017GL074593, 44, 15, 7766-7772, 2017.08, [URL], We investigate seismic wave velocity structure and spatial distribution of the seismicity in the source region of the 2016 Kumamoto earthquake sequence. A one-dimensional mean velocity shows that the seismogenic zone has a high-velocity and low-Vp/Vs ratio relative to the average velocity structure of Kyushu Island. This indicates that the crust is relatively strong, capable of sustaining sufficiently high strain energy to facilitate two large (Mj > 6.5) earthquakes in close proximity to one another in rapid succession. Three-dimensional tomography of the seismogenic zone around the source of the 2016 Kumamoto earthquake sequence yields Vp = 6 km/s and Vs = 3.5 km/s. Most large-displacement areas (asperities) of the Mj 7.3 event overlap with the seismogenic zone and the overlying surface layer. Aftershock seismicity is distributed deeper than the conventional seismogenic zone, which suggests decreased strength due to fluids or increased stress, both caused by coseismic slip..
5. Koki Aizawa, Hisafumi Asaue, Katsuaki Koike, Shinichi Takakura, Mitsuru Utsugi, Hiroyuki Inoue, Ryokei Yoshimura, Ken'Ichi Yamazaki, Shintaro Komatsu, Makoto Uyeshima, Takao Koyama, Wataru Kanda, Taro Shiotani, Nobuo Matsushima, Maki Hata, Tohru Yoshinaga, Kazunari Uchida, Yuko Tsukashima, Azusa Shito, Shiori Fujita, Asuma Wakabayashi, Kaori Tsukamoto, Takeshi Matsushima, Masahiro Miyazaki, Kentaro Kondo, Kanade Takashima, Takeshi Hashimoto, Makoto Tamura, Satoshi Matsumoto, Yusuke Yamashita, Manami Nakamoto, Hiroshi Shimizu, Seismicity controlled by resistivity structure
The 2016 Kumamoto earthquakes, Kyushu Island, Japan, Earth, Planets and Space, 10.1186/s40623-016-0590-2, 69, 1, 2017.01, [URL], The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50-100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005-3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL. [Figure not available: see fulltext.].
6. M. K. Savage, Y. Aoki, K. Unglert, T. Ohkura, K. Umakoshi, Hiroshi Shimizu, M. Iguchi, T. Tameguri, T. Ohminato, J. Mori, Stress, strain rate and anisotropy in Kyushu, Japan, Earth and Planetary Science Letters, 10.1016/j.epsl.2016.01.005, 439, 129-142, 2016.04, [URL], Seismic anisotropy, the directional dependence of wave speeds, may be caused by stress-oriented cracks or by strain-oriented minerals, yet few studies have quantitatively compared anisotropy to stress and strain over large regions. Here we compare crustal stress and strain rates on the Island of Kyushu, Japan, as measured from inversions of focal mechanisms, GPS and shear wave splitting. Over 85,000 shear wave splitting measurements from local and regional earthquakes are obtained from the NIED network between 2004 and 2012, and on Aso, Sakurajima, Kirishima and Unzen volcano networks. Strain rate measurements are made from the Japanese Geonet stations. JMA-determined S arrival times processed with the MFAST shear wave splitting code measure fast polarisations (Φ), related to the orientation of the anisotropic medium and time delays (dt), related to the path length and the percent anisotropy. We apply the TESSA 2-D delay time tomography and spatial averaging code to the highest quality events, which have nearly vertical incidence angles, separating the 3455 shallow (depth < 40 km) from the 4957 deep (> = 40 km) earthquakes. Using square grids with 30 km sides for all the inversions, the best correlations are observed between splitting from shallow earthquakes and stress. Axes of maximum horizontal stress (SHmax) and Φ correlate with a coefficient c of 0.56, significant at the 99% confidence level. Their mean difference is 31.9°. Axes of maximum compressional strain rate and SHmax are also well aligned, with an average difference of 28°, but they do not correlate with each other, meaning that where they differ, the difference is not systematic. Anisotropy strength is negatively correlated with the stress ratio parameter determined from focal mechanism inversion (c = -0.64; significant at the 99% confidence level). The anisotropy and stress results are consistent with stress-aligned microcracks in the crust in a dominantly strike-slip regime. Eigenvalues of maximum horizontal strain rate correlate positively with stress ratio (c = 0.43, significant at 99% confidence). All three orientations are E-W in central Kyushu, where the compressional strain rate is highest. Both splitting and stress suggest plate-boundary-parallel maximum principal stress just off the coast of Kyushu, where strain rate measurements are sparse. South western Kyushu has the largest difference between directions of strain rate and stress. Φ from shallow and deep earthquakes are not well aligned, suggesting that the deep earthquake waveforms are not simply split in the crust. Causes for the anisotropy may be olivine crystals aligned by drag of the subducting Philippine Sea plate in the mantle and stress-aligned microcracks in the crust..
7. Satoshi Matsumoto, Shigeru Nakao, Takahiro Ohkura, Masahiro Miyazaki, Hiroshi Shimizu, Yuki Abe, Hiroyuki Inoue, Manami Nakamoto, Shin Yoshikawa, Yusuke Yamashita, Spatial heterogeneities in tectonic stress in Kyushu, Japan and their relation to a major shear zone Seismology, Earth, Planets and Space, 10.1186/s40623-015-0342-8, 67, 1, 2015.12, [URL], We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region of the island, which is characterized by active volcanoes. We evaluated the focal mechanisms of the shallow earthquakes on Kyushu Island to determine the relative deviatoric stress field. Generally, the stress field was estimated by using the method proposed by Hardebeck and Michael (2006) for the strike-slip regime in this area. The minimum principal compression stress (σ3), with its near north-south trend, is dominant throughout the entire region. However, the σ 3 axes around the shear zone are rotated normal to the zone. This result is indicative of shear stress reduction at the zone and is consistent with the right-lateral fault behavior along the zone detected by a strain-rate field analysis with global positioning system data. Conversely, the stress field of the normal fault is dominant in the Beppu-Shimabara area, which is located in the central part of the island. This result and the direction of σ3 are consistent with the formation of a graben structure in the area..
8. Masahiro Miyazaki, Satoshi Matsumoto, Hiroshi Shimizu, Triggered tremors beneath the seismogenic zone of an active fault zone, Kyushu, Japan, Earth, Planets and Space, 10.1186/s40623-015-0346-4, 67, 1, 2015.12, [URL], Non-volcanic tremors were induced by the surface waves of the 2012 Sumatra earthquake around the Hinagu fault zone in Kyushu, Japan. We inferred from dense seismic observation data that the hypocenters of these tremors were located beneath the seismogenic zone of the Hinagu fault. Focal mechanisms of the tremors were estimated using S-wave polarization angles. The estimated focal mechanisms show similarities to those of shallow earthquakes in this region. In addition, one of the nodal planes of the focal mechanisms is almost parallel to the strike direction of the Hinagu fault. These observations suggest that the tremors were triggered at the deeper extension of the active fault zone under stress conditions similar to those in the shallower seismogenic region. A low-velocity anomaly beneath the hypocentral area of the tremors might be related to the tremor activity..
9. Y. Yamashita, H. Yakiwara, Y. Asano, Hiroshi Shimizu, K. Uchida, S. Hirano, K. Umakoshi, H. Miyamachi, M. Nakamoto, M. Fukui, M. Kamizono, H. Kanehara, T. Yamada, M. Shinohara, K. Obara, Migrating tremor off southern Kyushu as evidence for slow slip of a shallow subduction interface, Science, 10.1126/science.aaa4242, 348, 6235, 676-679, 2015.05, [URL], Detection of shallow slow earthquakes offers insight into the near-trench part of the subduction interface, an important region in the development of great earthquake ruptures and tsunami generation. Ocean-bottom monitoring of offshore seismicity off southern Kyushu, Japan, recorded a complete episode of low-frequency tremor, lasting for 1 month, that was associated with very-low-frequency earthquake (VLFE) activity in the shallow plate interface. The shallow tremor episode exhibited two migration modes reminiscent of deep tremor down-dip of the seismogenic zone in some other subduction zones: a large-scale slower propagation mode and a rapid reversal mode. These similarities in migration properties and the association with VLFEs strongly suggest that both the shallow and deep tremor and VLFE may be triggered by the migration of episodic slow slip events..
10. 九州大学地震火山観測研究センター, 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第117号, 170-171, 2015.03.
11. 九州大学地震火山観測研究センター , 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第117号, 182-185, 2015.03.
12. 中道治久, 筒井智樹, 為栗健, 井口正人, 八木原寛, 大湊隆雄, 菅井明, 大島弘光, 三浦哲, 山本希, 市來雅啓, 野上健治, 武尾実, 市原美恵, 及川純, 山中佳子, 大倉敬宏, 安部祐希, 清水 洋, 山下裕亮, 2013年桜島人工地震探査の概要と2008年探査との比較, 京都大学防災研究所年報, 第57号B, 125-137, 2014.06.
13. 筒井智樹, 井口正人, 中道治久, 為栗健, 八木原寛, 大湊隆雄, 菅井明, 大島弘光, 三浦哲, 山本希, 市來雅啓, 野上健治, 武尾実, 市原美恵, 及川純, 山中佳子, 大倉敬宏, 安部祐希, 清水 洋, 山下裕亮, 桜島火山における反復地震探査(2013年観測), 京都大学防災研究所年報, 第57号B, 138-149, 2014.06.
14. 九州大学地震火山観測研究センター , 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第114号, 161-162, 2014.03.
15. 九州大学地震火山観測研究センター , 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第114号, 169-173, 2014.03.
16. 九州大学地震火山観測研究センター, 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第115号, 119-120, 2014.03.
17. 九州大学地震火山観測研究センター , 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第115号, 127-131, 2014.03.
18. 九州大学地震火山観測研究センター, 阿蘇火山における地球化学的観測(6月~10月), 火山噴火予知連絡会会報, 第116号, 174-175, 2014.03.
19. 九州大学地震火山観測研究センター , 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第116号, 182-185, 2014.03.
20. 九州大学地震火山観測研究センター , 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第112号, 127-128, 2014.01.
21. 九州大学地震火山観測研究センター , 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第112号, 136-138, 2014.01.
22. 九州大学地震火山観測研究センター , 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第113号, 136-137, 2014.01.
23. 九州大学地震火山観測研究センター, 雲仙岳火山活動状況, 火山噴火予知連絡会会報, 第113号, 144-147, 2014.01.
24. 筒井智樹, 井口正人, 為栗健, 渡邊幸弘, 大島弘光, 植木貞人, 山本希, 豊国源知, 野上健治, 大湊隆雄, 及川純, 市原美恵, 中道治久, 大倉敬宏, 清水 洋, 宮町宏樹, 八木原寛, 園田忠臣, 高山鐡朗, 渡邉篤志, 桜島火山における反復地震探査(2012年観測), 京都大学防災研究所年報, 第56号B,207-220
, 2013.05.
25. Takeshi Akuhara, Kimihiro Mochizuki, Kazuo Nakahigashi, Tomoaki Yamada, Masanao Shinohara, Shin'Ichi Sakai, Toshihiko Kanazawa, Kenji Uehira, Hiroshi Shimizu, Segmentation of the Vp/Vs ratio and low-frequency earthquake distribution around the fault boundary of the Tonankai and Nankai earthquakes, Geophysical Research Letters, 10.1002/grl.50223, 40, 7, 1306-1310, 2013.04, [URL], Beneath the Kii Peninsula, the distribution of low-frequency earthquakes (LFEs) forms three clusters. A previous study shows that one of the clusters has anomalously less amount of cumulative slip than the others. To understand the cause of this variation, we applied a tomographic analysis using arrival times of earthquakes recorded by both ocean bottom seismometers and onshore stations. As a result, we identified segmentation about the Vp/Vs ratio around the subducting plate interface corresponding to the distribution of LFEs. One of the segments has low a Vp/Vs ratio that coincides with the small-slip LFE cluster. Another segment has a high Vp/Vs ratio in which almost no LFEs occur. We conclude that the relatively low pore fluid pressure within the low Vp/Vs segment contributes to the small-slip LFE cluster and that the gap of LFEs within the high Vp/Vs segment corresponds to stable slip area due to high pore fluid pressure. Key Points Segmentation of Vp/Vs ratio along subduction margin Spatial relationship between Vp/Vs ratio 3-D velocity structure from offshore to onshore around the Kii Peninsula.
26. Satoshi Matsumoto, Hiroshi Shimizu, Takeshi Matsushima, Kenji Uehira, Yusuke Yamashita, Manami Nakamoto, Masahiro Miyazaki, Hiromi Chikura, Short-term spatial change in a volcanic tremor source during the 2011 Kirishima eruption, earth, planets and space, 10.5047/eps.2012.09.002, 65, 4, 323-329, 2013.01, [URL], Volcanic tremors are indicators of magmatic behavior, which is strongly related to volcanic eruptions and activity. Detection of spatial and temporal variations in the source location is important for understanding the mechanism of volcanic eruptions. However, short-term temporal variations within a tremor event have not always been detected by seismic array observations around volcanoes. Here, we show that volcanic tremor sources were activated at both the top (i.e., the crater) and the lower end of the conduit, by analyzing seismograms from a dense seismic array 3 km from the Shinmoedake crater, Kirishima volcano, Japan. We observed changes in the seismic ray direction during a volcanic tremor sequence, and inferred two major sources of the tremor from the slowness vectors of the approaching waves. One was located in a shallow region beneath the Shinmoedake crater. The other was found in a direction N30°W from the array, pointing to a location above a pressure source. The fine spatial and temporal characteristics of volcanic tremors suggest an interaction between deep and shallow conduits..
27. Yusuke Yamashita, Hiroshi Shimizu, Kazuhiko Goto, Small repeating earthquake activity, interplate quasi-static slip, and interplate coupling in the Hyuga-nada, southwestern Japan subduction zone, Geophysical Research Letters, 10.1029/2012GL051476, 39, 8, 2012.04.
28. Matsumoto, S., K. Uehira, T. Matsushima, and H. Shimizu , 2012:Modeling heterogeneous deviatoric stress field around the hypocentral area of the 2005 Fukuoka earthquake (M7.0) by spatially distributed moment tensors, Journal of Geophysical Research, VOL. 117, B03303, doi:10.1029/2011JB008687, 2012.03.
29. Satoshi Matsumoto, Kenji Uehira, Takeshi Matsushima, Hiroshi Shimizu, Modeling heterogeneous deviatoric stress field around the hypocentral area of the 2005 Fukuoka earthquake (M7.0) by spatially distributed moment tensors, Journal of Geophysical Research: Solid Earth, 10.1029/2011JB008687, 117, 3, 2012.03, [URL], Recent studies on the stress field in a seismogenic zone revealed a heterogeneous feature around a fault. To model the stress field around a fault zone, we have developed an inversion method that can be applied to focal mechanism data on microearthquakes and slip data on faults. The modeling scheme takes an objective approach without an a priori model such as the existence of faults or a magma source. The stress variation resulting from inelastic deformation in a medium (e.g., fault slip and magma intrusion) can be expressed as equivalent body forces in the medium. Thus, we attempted to model the stress field through the estimation of parameters of the regional stress and spatially distributed moment sources. The method was applied to the focal mechanism data of the aftershocks of the 2005 Fukuoka earthquake (M7.0) in Japan. The direction obtained for the minimum regional principal stress (i.e., NNW-SSE) was as expected from the general tendency of the focal mechanisms. The results revealed that the stress field was distorted by the fault slip in the middle segment of the earthquake fault. The slip detected at the deep part of the fault is located away from the coseismic slip area, suggesting a possibility of either preseismic or postseismic slip around the initiation point of the main shock rupture. In contrast, the stress accumulation rises at the folding point of the aftershock alignment, and the stress relaxation area is found at the deeper edge where the largest aftershock occurred and strong heterogeneous medium exists..
30. Satoshi Matsumoto, Hiroshi Shimizu, Masazumi Onishi, Kenji Uehira, Seismic reflection survey of the crustal structure beneath Unzen volcano, Kyushu, Japan, earth, planets and space, 10.5047/eps.2011.11.006, 64, 5, 405-414, 2012.01, [URL], Unzen volcano is located in the western part of Kyushu, Japan. We carried out a seismic reflection survey at Unzen volcano in order to elucidate the structure of the volcano. Although the survey was conducted in a volcanic area under difficult conditions, such as artificial noises and a complex structure, we were able to resolve the structure beneath the profile using vibrator sources and a large number of stacking signals. The processed depth sections confirmed that Unzen volcano developed in a graben structure, as has been suggested in other geological studies. We imaged many subsurface normal faults shallower than 1 km. These faults, mostly covered with volcanic lava and deposits, were identified at the surface. Strong reflectors were found at a depth of approximately 3 km. They were located just above the pressure source of the latest eruption, as inferred from geodetic data. The geometric relationship between the reflection image, the pressure source location, and the lava dome suggests that the conduit from the lava dome could connect to the magma chamber located 4 km away from the lava dome..
31. 九州大学地震火山観測研究センター, 阿蘇火山における地球化学的観測, 火山噴火予知連絡会会報, 第106号, 122-124, 2011.11.
32. 九州大学地震火山観測研究センター, 雲仙火山活動状況(2010年2月〜2010年6月), 火山噴火予知連絡会会報, 第106号, 125-128, 2011.11.
33. 九州大学地震火山観測研究センター, 雲仙火山活動状況(2010年6月〜2010年10月), 火山噴火予知連絡会会報, 第107号, 150-153, 2011.11.
34. Kudo Umakoshi, Naoko Itasaka, Hiroshi Shimizu, High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, Vol.206, Issues 3-4 pages 70-79, 2011.09.
35. Kodo Umakoshi, Naoko Itasaka, Hiroshi Shimizu, High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2011.07.004, 206, 3-4, 70-79, 2011.09, [URL], The 1990-1995 eruption of Unzen Volcano in southwestern Japan was characterized by the extrusion of a dacite lava dome and frequent pyroclastic flows during the dome growth. Associated with the dome emergence on May 20, 1991, an intense swarm of high-frequency (HF) microearthquakes occurred just beneath the crater at very shallow depths. We used data from FG3, a seismic station located 500. m SSW of the crater, to identify 29,401 HF earthquakes between May 11 and 31, 1991. The rate of HF earthquakes increased starting on May 12 and peaked on May 17. The high seismicity continued until May 26, then dropped sharply, coinciding with a marked decline in the swelling on the southern upper flank of the volcano. The seismicity increased and decreased repeatedly within a 1- to 2-h period, which was correlated with tilt cycles observed 680. m west of the crater in such a way that the seismicity increased during uplifting on the craterward side. Defining an earthquake group as a series of earthquakes with waveforms that are similar or vary only slightly over time, we identified 10 such groups, each containing more than 300 events. The largest group comprised 3,214 events over 18. days. Seismicity rates of eight groups, including the largest, increased and decreased repeatedly, correlated with the tilt cycles. As the waveform data from station FG3 were considerably clipped for relatively large events, we analyzed data from two additional stations, KRA and CJA, located 3.5 and 8.7. km from the crater, respectively, and identified a total of five new groups. The activity of two groups recorded at KRA was correlated with the tilt cycles, but those of three groups recorded at CJA did not always show such a correlation. Hypocenters for the groups recorded at KRA were distributed to the east side of the conduit and a focal mechanism suggested that the events in this area occurred due to compressional stress produced by the inflation of the conduit. The groups recorded at CJA occurred to the north side of the dike trending westward from the conduit. The focal mechanisms have P-axes roughly trending to the dike, which can be explained by the compressional stress generated by the increasing thickness of the dike..
36. 筒井智樹,井口正人,為栗健,及川純,大島弘光,前川徳光,青山裕,植木貞人,平原聡,野上健治,大湊隆雄,市原美恵,辻浩,堀川信一郎,奥田隆,清水洋,松島健,大蔵敬宏,吉川慎,園田忠臣,宮町宏樹,八木原寛,平野舟一郎,斎藤公一滝,末峯宏一,後藤進,池亀孝光,加藤幸司,松末伸一,河野太亮,宇都宮真吾,五藤大仁,渡辺竜一,前原祐樹,佐藤泉,大薮竜童,清水英彦,山下裕亮, 桜島火山における反復地震探査(2010年観測), 京都大学防災研究所年報, 第54号B, 195-208, 2011.06.
37. 筒井智樹・井口正人・為栗 健・上田義浩・大島弘光・植木貞人・大湊隆雄・及川 純・市原美恵・野上健治・中道治久・大倉敬宏・清水 洋・宮町宏樹・八木原 寛・前川徳光・堀川信一郎・吉川 慎・園田忠臣・平野舟一郎・末峯宏一・林 乾太・加藤幸司・長尾 潤・池亀孝光・松末伸一・五藤大仁・河野太亮・梁田高広・田中窓香・渡辺竜一・長岡 優・前原祐樹・吉田沙由美・小林由美・栢橋志郎, 桜島火山における反復地震探査(一回目), 京都大学防災研究所年報, 第53号B, 241-259, 2010.10.
38. Saiga, A., S. Matsumoto, K. Uehira, T. Matsushima, and H. Shimizu, Velocity structure in the crust beneath the Kyushu area , Earth Planets Space, Vol. 62, No.5, 449-462, 2010.07.
39. Mochizuki, K., K. Nakahigashi, A. Kuwano, T. Yamada, M. Shinohara, S. Sakai, T. Kanazawa K. Uehira, and H. Shimizu, Seismic characteristics around the fault segment boundary of the historical great earthquakes along the Nankai Trough revealed by repeated long-term OBS observations, Geophysical Research Letters, 1029/2010GL042935, 37, 2010.05.
40. Kimihiro Mochizuki, Kazuo Nakahigashi, Asako Kuwano, Tomoaki Yamada, Masanao Shinohara, Shini'Chi Sakai, Toshihiko Kanazawa, Kenji Uehira, Hiroshi Shimizu, Seismic characteristics around the fault segment boundary of historical great earthquakes along the Nankai Trough revealed by repeated long-term OBS observations, Geophysical Research Letters, 10.1029/2010GL042935, 37, 9, 2010.05, [URL], The existence of a static fault segment boundary has been proposed for segmentation of the historical great earthquakes along the Nankai Trough, southwest of Japan. Due to the extremely low seismicity, the seismic characteristics around the boundary have remained too uncertain to allow detailed discussion of the cause of the fault segmentation. We collected four years of continuous seismic data around the segment boundary through repeated marine observations and determined accurate hypocenters, magnitudes and focal mechanisms of the observed earthquakes. The Tokai segment to the east of the boundary shows particularly low seismicity. An abrupt change in the P-axis orientation of intra-slab earthquakes coincides with heterogeneous structure within the subducting Philippine Sea Plate. The boundaries between regions of different seismic character are parallel to the magnetic anomaly lineation over the Shikoku Basin of the subducting plate, implying that they are determined by the formation process of the basin..
41. 清水 洋, 雲仙普賢岳, 砂防と治水, 43, 1, 77-78, 2010.04.
42. Matsumoto, S., K. Uehira, A. Watanabe, K. Goto, Y. Iio, N. Hirata, T. Okada, H. Takahashi, H. Shimizu, M. Shinohara and T. Kanazawa, High resolution Q-1 estimation based on extension of coda normalization method and its application to P–wave attenuation structure in the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0), 京都大学防災研究所地震予知研究センター研究成果集, 19, 73-88, 2010.03.
43. Atsushi Saiga, Satoshi Matsumoto, Kenji Uehira, Takeshi Matsushima, Hiroshi Shimizu, Velocity structure in the crust beneath the Kyushu area, earth, planets and space, 10.5047/eps.2010.02.003, 62, 5, 449-462, 2010.01, [URL], We present high-resolution three-dimensional tomographic images of the crust beneath the entire Kyushu arc, and particularly the western portion. Our results reveal a velocity anomaly that correlates well with an upper crustal gravity anomaly. Significant low-velocity anomalies exist beneath the Miyazaki plane and along the Beppu-Shimabara Graben. Another extensive low-velocity region near the bottom of the crust is located just below the volcanic front and between active volcanoes. The low-velocity anomalies exhibit low Vp and Vp/Vs characteristics, and the spatial relationship between these anomalies, the Bouguer gravity anomaly, and the Moho suggests that low-density material at the base of the crust is responsible for both the seismic and gravity signatures. We interpret this material to constitute a relict ridge subducting below the Kyushu Mountains..
44. S. Matsumoto, K. Uehira, A. Watanabe, K. Goto, Y. Iio, N. Hirata, T. Okada, H. Takahashi, H. Shimizu, M. Shinohara, T. Kanazawa, High resolution Q-1 estimation based on extension of coda normalization method and its application to P-wave attenuation structure in the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0), Geophysical Journal International, 10.1111/j.1365-246X.2009.04313.x, 179, 2, 1039-1054, 2009.11, [URL], We developed a method for estimating the seismic-wave attenuation (1/Q) in a seismically active region such as an aftershock area. To estimate the attenuation factor between two adjacent hypocentres, we employed two seismograms recorded at a station and calculate the ratio between two power spectra of direct waves normalized by those in the coda for both the events. The coda normalization of the spectrum and the ratio between the two events minimize the possible influences of sources, sites, instruments and attenuation from the station to the hypocentral area. The 1/Q value can be estimated from the variation of the ratios of event pairs calculated at many stations for various traveltime differences between the even pairs. This method involves double-difference (DD) estimation from the logarithmic power spectra for estimating the 1/Q structure, employing a concept similar to the recently developed 'DD tomography' for velocity structures. We applied this method to the aftershock area of the 2005 West Off Fukuoka Prefecture Earthquake (M7.0) in order to investigate the 1/Q structure of P waves. By using the spectra of the seismograms of 1781 events recorded at a dense seismic network deployed around the aftershock area, we obtained the spatial variation of the 1/Q value in this region. The 1/Q distribution thereby obtained suggests that there exists a high-attenuation region around the edge of the main shock fault, which could correspond to the segment boundary between the earthquake fault and the adjacent faults. In addition, we found that the initiation points of the main shock and aftershocks were located in a low-attenuation region..
45. 清水 洋, 噴火の脅威と噴火災害軽減のための取り組み, 日本機械学会誌, Vol.112,No.1091,18-21, 2009.10.
46. 福井理作・清水 洋, 阿蘇栃木・垂玉温泉における二酸化炭素・水温観測, 第4回阿蘇火山の集中総合観測−2008年4月〜2009年3月−, 157-164, 2009.07.
47. 井口正人・為栗 健・山本圭吾・大島弘光・前川徳光・森 済・鈴木敦生・筒井智樹・今井幹浩・對馬和希・八木直史・植木貞人・中山貴史・山本芳裕・高木涼太・猪井志織・古賀祥子・西村太志・Titi ANGGONO・山本 希・及川 純・長田 昇・市原美恵・辻 浩・青木陽介・森田裕一・渡邊篤志・野上健治・山脇輝夫・渡辺俊樹・中道治久・奥田 隆・立花健二・Enrique HERNANDEZ・橋田悠・平井 敬・吉本昌弘・山崎賢志・毛利拓治・清水 洋・中元真美・山下裕亮・三ケ田均・尾西恭亮・田中 暁・岡野 豊・川林徹也・藤谷淳司・坂口弘訓・今泉光智哲・大倉敬宏・吉川 慎・安部祐希・安藤隆志・横尾亮彦・相澤広記・髙山鐵朗・山崎友也・多田光弘・市川信夫・加茂正人・富阪和秀・宮町宏樹・小林励司・八木原寛・平野舟一郎・泊 知里・西山信吾・吉田沙由美・畠山謙吾・西田 誠・加藤幸司・宮村淳一・小枝智幸・増田与志郎・平松秀行・河野太亮・松末伸一・大薄富士男・五藤大仁・宮下 誠・伊藤弘志・音成陽二郎, 2008年桜島人工地震探査の目的と実施, 京都大学防災研究所年報, 52, 293-307, 2009.06.
48. 河野裕希・松本 聡・松島 健・植平賢司・清水 洋・馬越孝道, 雲仙火山周辺域における相対応力場と1990-1995年噴火活動, 北海道大学地球物理学研究報告, 第72号,363-371, 2009.03.
49. Machida, Y., M. Shinohara, T. Takanami, Y. Murai, T. Yamada, N. Hirata, K. Suyehiro, T. Kanazawa, Y. Kaneda, H. Mikada, S. Sakai, T. Watanabe, K. Uehira, N. Takahashi, M. Nishino, K. Mochizuki, T. Sato, E. Araki, R. Hino, K. Uhira, H. Shiobara, and H. Shimizu, Heterogeneous structure around the rupture area of the 2003 Tokachi-oki earthquake (Mw=8.0), Japan, as revealed by aftershock observations using Ocean Bottom Seismometers, Tectonophysics, Vol.465, 164-176, 2009.02.
50. Yuya Machida, Masanao Shinohara, Tetsuo Takanami, Yoshio Murai, Tomoaki Yamada, Naoshi Hirata, Kiyoshi Suyehiro, Toshihiko Kanazawa, Yoshiyuki Kaneda, Hitoshi Mikada, Shin'ichi Sakai, Tomoki Watanabe, Kenji Uehira, Narumi Takahashi, Minoru Nishino, Kimihiro Mochizuki, Takeshi Sato, Ei'ichiro Araki, Ryota Hino, Kouichi Uhira, Hajime Shiobara, Hiroshi Shimizu, Heterogeneous structure around the rupture area of the 2003 Tokachi-oki earthquake (Mw = 8.0), Japan, as revealed by aftershock observations using Ocean Bottom Seismometers, Tectonophysics, 10.1016/j.tecto.2008.11.009, 465, 1-4, 164-176, 2009.02, [URL], Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw = 8.0), occurred on September 26, 2003. In order to investigate aftershock activity in the rupture area, 47 Ocean Bottom Seismometers (OBSs) were quickly deployed after the main shock. In the present study, we simultaneously estimate the hypocenters and 3-D seismic velocity models from the P- and S-wave arrivals of the aftershocks recorded by OBSs. The subducting plate is clearly imaged as a northwest dipping zone in which Vp is greater than 7 km/s, and the relocated hypocenters also show the subducting Pacific Plate. The aftershock distribution reveals that the dip angle of the plate boundary increases abruptly around 90 km from the Kuril Trench. The bending of the subducting plate corresponds to the southeastern edge of the rupture area. The island arc crust on the overriding plate has P-wave velocities of 6-7 km/s and a Vp/Vs of 1.73. A region of Vp/Vs greater than 1.88 was found north of the epicenter of the main shock. The depth of the high Vp/Vs region extends about 10 km upward from the plate interface. The plate boundary just below the high Vp/Vs region has the largest slip at the main rupture. A high Vp anomaly (~ 7.5 km/s) is found in the island arc crust in northeast part of the study area, which we interpret as a structural boundary related to the arc-arc collisional tectonics of the Hokkaido region, as the rupture of the main shock terminated at this high Vp region. We suggest that the plate interface geometry and the trench-parallel velocity heterogeneity in the landward plate are principal factors in controlling the rupture area of the main shock..
51. 河野裕希・松本 聡・松島 健・植平賢司・清水 洋・馬越孝道, 1990〜1995年雲仙火山噴火前に起きた応力場の変化, 月刊地球, 号外,No.60,85-90, 2008.12.
52. Kohno, Y., T. Matsushima, H. Shimizu, Pressure sources beneath Unzen Volcano inferred from leveling and GPS data, Journal of Volcanology and Geothermal Research, Vol.175, 100-109, 2008.07.
53. Umakoshi, K., N. Takamura, N. Shinzato, K. Uchida, N. Matsuwo, H. Shimizu, Seismicity associated with the 1991-1995 dome growth at Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, Vol.175, 91-99, 2008.07.
54. Sakuma, S., T. Kajiwara, S. Nakada, K. Uto and H. Shimizu, Drilling and logging results of USDP-4 − Penetration into the volcanic conduit of Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, Vol.175, 1-12, 2008.07.
55. Sumio Sakuma, Tatsuya Kajiwara, Setsuya Nakada, Kozo Uto, Hiroshi Shimizu, Drilling and logging results of USDP-4 - Penetration into the volcanic conduit of Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.039, 175, 1-2, 1-12, 2008.07, [URL], Borehole USDP-4 was drilled into the Unzen volcanic conduit 9 years after its last eruption. The sub-surface mechanism of eruption at Unzen Volcano was investigated by taking cores and by geophysical logging. The drilling operations were carried out in 2003-04, as a joint research program sponsored by the Japanese Government and the International Continental Scientific Drilling Program (ICDP). Borehole USDP-4 was a directional 6-1/4 in. (158.8 mm) borehole drilled from a site located on the northern slope of the volcano at 840 m above sea level, and was designed to penetrate into the conduit at sea level with a final deviation angle of over 70° from vertical and a borehole depth of 1800 m. An igneous dyke with a high likelihood of being the Unzen conduit was encountered at 1996 m depth and core samples were taken from that location. Chemical analysis of drill cores confirmed the identification of the conduit. Geophysical logging, including recording of gamma ray, resistivity, self-potential, density, neutron porosity, sonic velocity, and temperature suggested an alternation of dykes and flows in rock penetrated during the drilling, and in-hole pictures confirmed the lithologic identifications. Although borehole collapse and high temperature had been expected in the conduit, the actual drilling and logging into the conduit experienced no gas or fluid kick, and the measured temperatures within the dyke were below 200 °C..
56. Sumio Sakuma, Tatsuya Kajiwara, Setsuya Nakada, Kozo Uto, Hiroshi Shimizu, Drilling and logging results of USDP-4 - Penetration into the volcanic conduit of Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.039, 175, 1-2, 1-12, 2008.07, [URL], Borehole USDP-4 was drilled into the Unzen volcanic conduit 9 years after its last eruption. The sub-surface mechanism of eruption at Unzen Volcano was investigated by taking cores and by geophysical logging. The drilling operations were carried out in 2003-04, as a joint research program sponsored by the Japanese Government and the International Continental Scientific Drilling Program (ICDP). Borehole USDP-4 was a directional 6-1/4 in. (158.8 mm) borehole drilled from a site located on the northern slope of the volcano at 840 m above sea level, and was designed to penetrate into the conduit at sea level with a final deviation angle of over 70° from vertical and a borehole depth of 1800 m. An igneous dyke with a high likelihood of being the Unzen conduit was encountered at 1996 m depth and core samples were taken from that location. Chemical analysis of drill cores confirmed the identification of the conduit. Geophysical logging, including recording of gamma ray, resistivity, self-potential, density, neutron porosity, sonic velocity, and temperature suggested an alternation of dykes and flows in rock penetrated during the drilling, and in-hole pictures confirmed the lithologic identifications. Although borehole collapse and high temperature had been expected in the conduit, the actual drilling and logging into the conduit experienced no gas or fluid kick, and the measured temperatures within the dyke were below 200 °C..
57. Yuhki Kohno, Takeshi Matsushima, Hiroshi Shimizu, Pressure sources beneath Unzen Volcano inferred from leveling and GPS data, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.022, 175, 1-2, 100-109, 2008.07, [URL], Unzen Volcano erupted from 1990 to 1995 and the resultant volcanic deformations were measured by GPS, leveling, tilt meter, and EDM. The largest vertical displacement observed was subsidence of 8 cm in the western part of Shimabara Peninsula. Numerous magma chamber models were proposed based on the geodesic data obtained. However, these models could not explain the volcanic deformations observed around Chijiwa Bay after the eruption had finished because previous studies only used data collected during the eruption. We therefore re-examined the magma chamber model using leveling and GPS data, and included measurements collected both during and after the eruption on leveling. We applied the Mogi Model with a height correction applied to the measuring points. There are four parameters for each source: longitude, latitude, depth, and volume change. Each source parameter was estimated using a grid-search scheme. We also used AIC (Akaike Information Criterion) to fix the number of pressure sources beneath Unzen to explain the vertical and horizontal deformations during 1986-2004. The results show that there are four pressure sources under Unzen Volcano and that magma ascended obliquely at an angle of about 45° from beneath Chijiwa Bay to the crater created by this eruption. After the eruption had stopped, magma inflows into deep sources were identified based on our leveling survey conducted after 1996. We compare our source model with several seismic data sets. Before the Unzen eruption, an earthquake swarm occurred from 1989 to 1990 in which pressure sources were detected just below the earthquake hypocenters. Other seismic refraction studies support our estimates of the location of these pressure sources. Using volume change values we inferred that magma supply had continued into this deep source at the rate of 2.0-~ 2.5 m3/yr from at least 1984 until after eruption stopped..
58. Yuhki Kohno, Takeshi Matsushima, Hiroshi Shimizu, Pressure sources beneath Unzen Volcano inferred from leveling and GPS data, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.022, 175, 1-2, 100-109, 2008.07, [URL], Unzen Volcano erupted from 1990 to 1995 and the resultant volcanic deformations were measured by GPS, leveling, tilt meter, and EDM. The largest vertical displacement observed was subsidence of 8 cm in the western part of Shimabara Peninsula. Numerous magma chamber models were proposed based on the geodesic data obtained. However, these models could not explain the volcanic deformations observed around Chijiwa Bay after the eruption had finished because previous studies only used data collected during the eruption. We therefore re-examined the magma chamber model using leveling and GPS data, and included measurements collected both during and after the eruption on leveling. We applied the Mogi Model with a height correction applied to the measuring points. There are four parameters for each source: longitude, latitude, depth, and volume change. Each source parameter was estimated using a grid-search scheme. We also used AIC (Akaike Information Criterion) to fix the number of pressure sources beneath Unzen to explain the vertical and horizontal deformations during 1986-2004. The results show that there are four pressure sources under Unzen Volcano and that magma ascended obliquely at an angle of about 45° from beneath Chijiwa Bay to the crater created by this eruption. After the eruption had stopped, magma inflows into deep sources were identified based on our leveling survey conducted after 1996. We compare our source model with several seismic data sets. Before the Unzen eruption, an earthquake swarm occurred from 1989 to 1990 in which pressure sources were detected just below the earthquake hypocenters. Other seismic refraction studies support our estimates of the location of these pressure sources. Using volume change values we inferred that magma supply had continued into this deep source at the rate of 2.0-~ 2.5 m3/yr from at least 1984 until after eruption stopped..
59. K. Umakoshi, N. Takamura, N. Shinzato, K. Uchida, N. Matsuwo, H. Shimizu, Seismicity associated with the 1991-1995 dome growth at Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.030, 175, 1-2, 91-99, 2008.07, [URL], From May 1991 until February 1995, seismicity in the crater area of Unzen Volcano, southwest Japan, intensified in conjunction with the growth of a dacite lava dome. We used data from seismic stations located near the crater to identify approximately 580,000 summit earthquakes with maximum amplitudes equal to or greater than 1 × 10- 3 cm/s. The temporal characteristics of the seismicity level were different for exogenous and endogenous periods of dome growth. Periods of solely exogenous growth were accompanied by several days or weeks of increased seismicity, and levels of seismicity were notably reduced between successive seismically active periods. In contrast, levels of seismicity were generally high during periods when the dome grew endogenously, with repeated cycles of increasing and decreasing seismicity of one to two months duration. We classified the waveforms of summit earthquakes into high-frequency (HF), medium-frequency (MF), and low-frequency (LF) types on the basis of spectral analysis. Dominant waveform types varied significantly over time: HF was dominant in May 1991, LF from June 1991 until August 1993, MF during September and October 1993, and HF and MF from November 1993. HF and MF events are mainly distributed at depths of 500-1100 m above sea level (ASL), just below the lava dome, while LF events are widely distributed at depths from 500 m ASL to the interior of the dome itself. To efficiently detect earthquake families, we conducted a cross-correlation analysis of waveforms. Using the events one-by-one as reference events, we calculated peak correlation coefficients between each reference event and events that occurred within 24 h either side of the reference event. The results show that many earthquake families of all waveform types occurred throughout the growth period of the dome. The durations of most families were less than two weeks and were not related to the intensity of seismic activity. The incidence rate of events within each family reached a peak in the middle of the respective activity period. The results also suggest that the waveforms in several analyzed families gradually evolved over time. During periods when HF events intensified, several families appeared contemporaneously. In contrast, during periods when LF events intensified, the occurrence pattern of earthquake families was relatively simple: a new family became active only once the activity of the previous family had declined or ceased completely. A possible source mechanism for families of HF events is stick-slip within the stiff rocks surrounding the conduit; however, several different types of source mechanisms should be considered for families of LF events..
60. K. Umakoshi, N. Takamura, N. Shinzato, K. Uchida, N. Matsuwo, H. Shimizu, Seismicity associated with the 1991-1995 dome growth at Unzen Volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/j.jvolgeores.2008.03.030, 175, 1-2, 91-99, 2008.07, [URL], From May 1991 until February 1995, seismicity in the crater area of Unzen Volcano, southwest Japan, intensified in conjunction with the growth of a dacite lava dome. We used data from seismic stations located near the crater to identify approximately 580,000 summit earthquakes with maximum amplitudes equal to or greater than 1 × 10- 3 cm/s. The temporal characteristics of the seismicity level were different for exogenous and endogenous periods of dome growth. Periods of solely exogenous growth were accompanied by several days or weeks of increased seismicity, and levels of seismicity were notably reduced between successive seismically active periods. In contrast, levels of seismicity were generally high during periods when the dome grew endogenously, with repeated cycles of increasing and decreasing seismicity of one to two months duration. We classified the waveforms of summit earthquakes into high-frequency (HF), medium-frequency (MF), and low-frequency (LF) types on the basis of spectral analysis. Dominant waveform types varied significantly over time: HF was dominant in May 1991, LF from June 1991 until August 1993, MF during September and October 1993, and HF and MF from November 1993. HF and MF events are mainly distributed at depths of 500-1100 m above sea level (ASL), just below the lava dome, while LF events are widely distributed at depths from 500 m ASL to the interior of the dome itself. To efficiently detect earthquake families, we conducted a cross-correlation analysis of waveforms. Using the events one-by-one as reference events, we calculated peak correlation coefficients between each reference event and events that occurred within 24 h either side of the reference event. The results show that many earthquake families of all waveform types occurred throughout the growth period of the dome. The durations of most families were less than two weeks and were not related to the intensity of seismic activity. The incidence rate of events within each family reached a peak in the middle of the respective activity period. The results also suggest that the waveforms in several analyzed families gradually evolved over time. During periods when HF events intensified, several families appeared contemporaneously. In contrast, during periods when LF events intensified, the occurrence pattern of earthquake families was relatively simple: a new family became active only once the activity of the previous family had declined or ceased completely. A possible source mechanism for families of HF events is stick-slip within the stiff rocks surrounding the conduit; however, several different types of source mechanisms should be considered for families of LF events..
61. 2003年九州日奈久断層域構造探査グループ, 九州日奈久断層域における地殻構造探査, 東京大学地震研究所彙報, 第83号,第1冊,103-130, 2008.06.
62. Tahara, M., K. Uehira, H. Shimizu, M. Nakada, T. Yamada, K. Mochizuki, M. Shinohara, M. Nishino, R. Hino, H. Yakiwara, H. Miyamachi, K. Umakoshi, M. Goda, N. Matsuwo and T. Kanazawa, Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift, Physics of the Earth and Planetary Interiors, 167, 19-33, 2008.04.
63. M. Tahara, K. Uehira, H. Shimizu, M. Nakada, T. Yamada, K. Mochizuki, M. Shinohara, M. Nishino, R. Hino, H. Yakiwara, H. Miyamachi, K. Umakoshi, M. Goda, N. Matsuwo, T. Kanazawa, Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift, Physics of the Earth and Planetary Interiors, 10.1016/j.pepi.2008.02.001, 167, 1-2, 19-33, 2008.03, [URL], The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of ∼120 m during the past ∼120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of -130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25-35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region..
64. M. Tahara, K. Uehira, H. Shimizu, M. Nakada, T. Yamada, K. Mochizuki, M. Shinohara, M. Nishino, R. Hino, H. Yakiwara, H. Miyamachi, K. Umakoshi, M. Goda, N. Matsuwo, T. Kanazawa, Seismic velocity structure around the Hyuganada region, Southwest Japan, derived from seismic tomography using land and OBS data and its implications for interplate coupling and vertical crustal uplift, Physics of the Earth and Planetary Interiors, 10.1016/j.pepi.2008.02.001, 167, 1-2, 19-33, 2008.03, [URL], The Hyuganada region, a forearc region of Southwest Japan, is characterized by several interesting geological and geophysical features, i.e., significant aseismic crustal uplift of ∼120 m during the past ∼120 thousand years at the Miyazaki Plain, negative free-air gravity anomalies with the maximum magnitude of -130 mgal, and relatively less cohesive interplate coupling compared with that for off the Shikoku and Kii Peninsula. In order to examine the causes of these observations, we determined a detailed three-dimensional seismic velocity structure based on the seismic data observed by ocean bottom seismometers (OBS) and land stations. P- and S-wave tomographic velocity structures clearly indicate the subducting slab and also the zones of high Poisson's ratio at 25-35 km depth along the coastline of the northeastern part of the Hyuganada. The region with high Poisson's ratio may correspond to the serpentinized mantle wedge as suggested for other mantle wedges, and appears to be coincident with the zone for observed aseismic slips such as the slow-slip and after-slip events. Also, the detection may be related to a relatively weak interplate coupling in the Hyuganada region. The tomographic structures also indicate low velocity zones with a horizontal scale comparable to the Kyushu-Palau Ridge in and around the subducting slab. If we assume that the low velocity zones correspond to the subducted Kyushu-Palau Ridge, then the predicted gravity anomaly due to the density contrast between the low velocity zones and the surrounding region can explain about 60% of the gravity anomaly in the Hyuganada region. The buoyancy is probably an important factor for the crustal uplift observed in the Miyazaki Plain, the steep bending of the subducting slab and the normal fault-type earthquakes around the Hyuganada region..
65. 渡辺秀文・中道治久・大湊隆雄・鍵山恒臣・及川 純・青木陽介・辻  浩・小山悦郎・長田 昇・萩原道徳・竹田豊太郎・小林知勝・大島弘光・青山 裕・前川徳光・鈴木敦生・田中 聡・西村太志・仁田交市・山崎 純・奥田 隆・山田 守・大倉敬宏・須藤靖明・吉川 慎・井口正人・清水 洋・植平賢司・松島 健・八木原 寛・平野舟一郎, 富士山の大学合同稠密地震観測(2002年9月-2005年4月), 東京大学地震研究所彙報, 第82号,第3冊,195-207, 2008.02.
66. 第5回火山都市国際会議実行委員会, 第5回火山都市国際会議, 火山, 53, 51-62, 2008.02.
67. 清水 洋・松本 聡・河野裕希・松島 健・植平賢司, 福岡西方沖地震のその後 -来るべき警固断層地震へむけて-, 長崎県地学会誌, 71, 39-40, 2007.10.
68. 及川 純・鍵山恒臣・田中 聡・宮町宏樹・筒井智樹・池田 靖・潟山弘明・松尾紃道・大島弘光・西村裕一・山本圭吾・渡辺俊樹・山崎文人・渡辺秀文・藤井敏嗣・中田節也・武尾 実・, 富士山における人工地震探査−観測および走時の読み取り−, 東京大学地震研究所彙報, 第81号,第1冊,71-94, 2007.03.
69. 清水 洋, 特集:2005年福岡県西方沖の地震 (M7.0), 地震第2輯, 第59巻,第3号,239-240, 2007.03.
70. 清水 洋, 2005年福岡県西方沖地震(M7.0)-総論-, 月刊地球, Vol.29, No.2, 91-97, 2007.02.
71. 植平賢司,山田知朗,篠原雅尚,中東和夫,宮町宏樹,飯尾能久,岡田知巳,高橋浩晃,松尾のり道,内田和也,金沢敏彦,清水 洋, 福岡県西方沖地震の精密震源分布と発震機構, 月刊地球, Vol.29,No.2,91-97, 2007.02.
72. 飯尾能久,片尾 浩,上野友岳,Bogdan Enescu,平野憲雄,岡田知巳,内田直希,植平賢司,松本 聡,松島 健,清水 洋, 福岡県西方沖地震の余震の応力降下量の空間分布, 月刊地球, Vol.29,No.2,123-127, 2007.02.
73. 齋藤政城・松島 健・松尾のり道・清水 洋, 阿蘇中岳火山の二酸化硫黄および二酸化炭素ガス放出量の測定, 九州大学大学院理学研究院 研究報告 地球惑星科学, 第22巻,第2号,51-62, 2007.02.
74. Masaki Saito, Takeshi Matsushima, Norimichi Matsuwo, Hiroshi Shimizu, Observation SO2 and CO2 fluxes in and around the active crater of Aso Nakadake Volcano, Science Reports of the Kyushu University, Department of Earth and Planetary Sciences, 22, 2, 51-62, 2007.02, Volatiles of the magma such as H2O, CO2, SO2, H2S and HC1, which are the main components of volcanic explosions, discharge to the surface of the earth as volcanic gases, hot springs, and river waters. The discharge occurs not only in volcanic eruptions but also in non-erupting activities. In order to clarify the mass balance of volatiles, the hydrothermal structure and the volcanic activities, it is very important to characterize the chemical constituents and the amount of the volatiles discharged from volcanoes. It is thought that the volcanic gases reflect immediately the changes of magmatic activity because the mobility of gases is the most abundant among the discharge forms of volatiles. The diffuse degassing from soil is one of the discharge forms paid attention in recent years. As CO2 is hardly influenced by the groundwater due to low solubility in water, the direct information on volcanic activities can be obtained by observing the CO2 flux and its changes. Aso volcano is one of the most active volcanoes in Japan, and magmatic eruptions have occurred several times in historical times. However, the research on the amount of the CO2 flux from the soil in and around the active craters had not been performed at Aso volcano. In this study, we estimated the CO2 fluxes from the Nakadake crater and the circumference soil at Aso volcano and compared the results with the CO2 fluxes at Kusatsu-Shirane volcano. At Aso volcano, the amount of the total CO2 flux from soil was estimated to be about 0.12 ton d-1. While the amount of CO2 flux from the Nakadake First crater was estimated to be 597 ± 95 ton d-1, and thus the CO2 flux from the crater was much larger than that from the soil at Aso volcano. On the contrary, the CO2 flux from the soil was comparable to that from the craters (fumaroles) at Kusatsu-Shirane volcano..
75. Masaki Saito, Takeshi Matsushima, Norimichi Matsuwo, Hiroshi Shimizu, Observation SO2 and CO2 fluxes in and around the active crater of Aso Nakadake Volcano, Science Reports of the Kyushu University, Department of Earth and Planetary Sciences, 22, 2, 51-62, 2007.02, Volatiles of the magma such as H2O, CO2, SO2, H2S and HC1, which are the main components of volcanic explosions, discharge to the surface of the earth as volcanic gases, hot springs, and river waters. The discharge occurs not only in volcanic eruptions but also in non-erupting activities. In order to clarify the mass balance of volatiles, the hydrothermal structure and the volcanic activities, it is very important to characterize the chemical constituents and the amount of the volatiles discharged from volcanoes. It is thought that the volcanic gases reflect immediately the changes of magmatic activity because the mobility of gases is the most abundant among the discharge forms of volatiles. The diffuse degassing from soil is one of the discharge forms paid attention in recent years. As CO2 is hardly influenced by the groundwater due to low solubility in water, the direct information on volcanic activities can be obtained by observing the CO2 flux and its changes. Aso volcano is one of the most active volcanoes in Japan, and magmatic eruptions have occurred several times in historical times. However, the research on the amount of the CO2 flux from the soil in and around the active craters had not been performed at Aso volcano. In this study, we estimated the CO2 fluxes from the Nakadake crater and the circumference soil at Aso volcano and compared the results with the CO2 fluxes at Kusatsu-Shirane volcano. At Aso volcano, the amount of the total CO2 flux from soil was estimated to be about 0.12 ton d-1. While the amount of CO2 flux from the Nakadake First crater was estimated to be 597 ± 95 ton d-1, and thus the CO2 flux from the crater was much larger than that from the soil at Aso volcano. On the contrary, the CO2 flux from the soil was comparable to that from the craters (fumaroles) at Kusatsu-Shirane volcano..
76. Uehira, K., T. Yamada, M. Shinohara, K. Nakahigashi, H. Miyamachi, Y. Iio, T. Okada, H. Takahashi, N. Matsuwo, K. Uchida, T. Kanazawa, and H. Shimizu, Precise aftershock distribution of the 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network, Earth Planets Space, Vol.58, No.12, 1605-1610, 2006.12.
77. 清水 洋, 福岡県西方沖地震の余震活動と震源断層の構造, 西部地区自然災害資料センターニュース, No.35, 3-8, 2006.06.
78. Michitaka Tahara, Hiroshi Shimizu, Masao Nakada, Yoshihiro Ito, Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan, Physics of the Earth and Planetary Interiors, 10.1016/j.pepi.2005.12.004, 155, 3-4, 219-235, 2006.05, [URL], The recurrence interval and magnitude of great interplate earthquakes and characteristics of crustal uplift in the Late Pleistocene indicate clear lateral variations of the plate interaction on the convergent boundary along the Nankai Trough, i.e. relatively less cohesive for the Hyuganada region than for off the Shikoku and Kii Peninsula. The Hyuganada region is characterized by both significant aseismic crustal uplift in the Late Pleistocene and negative free-air gravity anomalies with the maximum magnitude of ∼-130 mgal. In order to examine the relationship for these observations, we investigated the focal depth distribution in the Hyuganada region by using sP depth phases. The focal depth distribution obtained by sP depth phases indicates different seismicity patterns in the southern and northern regions of the forearc. That is, most events for the southern and northern areas are thrust type and normal fault type, respectively. In the central part of the Hyuganada region located in the central area of negative gravity anomalies, however, each type event occurs in clusters and there is a seismicity gap between each cluster. The hypocenters for thrust type events generally coincide with the plate boundary as usual cases. However, most normal fault type events, with hypocenters in the landward side for the thrust type events and T-axes parallel to the plate boundary, occur in the crust or around the plate boundary of the forearc and the epicenters coincide with the peak position of negative gravity anomalies. The correlation between the seismicity and gravity anomaly suggests that the buoyancy inferred from negative gravity anomalies may cause the observed tensile events and a few relatively small asperities of the observed Mw 7 class earthquakes on the convergent boundary. This may be related to relatively less cohesive coupling around the Hyuganada region compared with that for off the Shikoku..
79. Michitaka Tahara, Hiroshi Shimizu, Masao Nakada, Yoshihiro Ito, Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan, Physics of the Earth and Planetary Interiors, 10.1016/j.pepi.2005.12.004, 155, 3-4, 219-235, 2006.05, [URL], The recurrence interval and magnitude of great interplate earthquakes and characteristics of crustal uplift in the Late Pleistocene indicate clear lateral variations of the plate interaction on the convergent boundary along the Nankai Trough, i.e. relatively less cohesive for the Hyuganada region than for off the Shikoku and Kii Peninsula. The Hyuganada region is characterized by both significant aseismic crustal uplift in the Late Pleistocene and negative free-air gravity anomalies with the maximum magnitude of ∼-130 mgal. In order to examine the relationship for these observations, we investigated the focal depth distribution in the Hyuganada region by using sP depth phases. The focal depth distribution obtained by sP depth phases indicates different seismicity patterns in the southern and northern regions of the forearc. That is, most events for the southern and northern areas are thrust type and normal fault type, respectively. In the central part of the Hyuganada region located in the central area of negative gravity anomalies, however, each type event occurs in clusters and there is a seismicity gap between each cluster. The hypocenters for thrust type events generally coincide with the plate boundary as usual cases. However, most normal fault type events, with hypocenters in the landward side for the thrust type events and T-axes parallel to the plate boundary, occur in the crust or around the plate boundary of the forearc and the epicenters coincide with the peak position of negative gravity anomalies. The correlation between the seismicity and gravity anomaly suggests that the buoyancy inferred from negative gravity anomalies may cause the observed tensile events and a few relatively small asperities of the observed Mw 7 class earthquakes on the convergent boundary. This may be related to relatively less cohesive coupling around the Hyuganada region compared with that for off the Shikoku..
80. 井口正人・八木原 寛・為栗 健・清水 洋・平林順一・宮町宏樹・鈴木敦生・筒井智樹・及川 順・森 健彦・相沢広記・河野裕希・馬場龍太・大倉敬宏・吉川 慎・齋藤武士・福嶋麻沙, 諏訪瀬島火山における人工地震探査, 京都大学防災研究所年報, 第49号B,339-352, 2006.01.
81. Tahara, M., H. Shimizu, M. Nakada, Y. Ito, Focal depth distribution using sP depth phase and implications for plate coupling in the Hyuganada region, Japan, Physics of the Earth and Planetary Interiors, 155, 219-235, 2006.01.
82. Watanabe, A., S. Matsumoto, T. Matsushima, K. Uehira, N. Matsuwo, and H. Shimizu, Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake, Earth Planets Space, Earth Planets Space, Vol.58, No.12, 1633-1636, 2006.01.
83. Hori, M., S. Matsumoto, K. Uehira, T. Okada, T. Yamada, Y. Iio, M. Shinohara, H. Miyamachi, H. Takahashi, K. Nakahigashi, A. Watanabe, T. Matsushima, N. Matsuwo, T. Kanazawa, and H. Shimizu, Three-dimensional seismic velocity structure as determined by double-difference tomography in and around the focal area of the 2005 West off Fukuoka Prefecture earthquake, Earth Planets Space, Vol.58, No.12, 1621-1626, 2006.01.
84. Iio, Y., H. Katao, T. Ueno, B. Enescu, N. Hirano, T. Okada, N. Uchida, S. Matsumoto, T. Matsushima, K. Uehira, and H. Shimizu, Spatial distribution of static stress drops for aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, Earth Planets Space, Earth Planets Space, Vol.58, No.12, 1611-1615, 2006.01.
85. 清水 洋, 長崎県において想定される被害地震, 長崎県地学会誌, 第70号,51, 2006.01.
86. Shimizu, H., H. Takahashi, T. Okada, T. Kanazawa, Y. Iio, H. Miyamachi, T. Matsushima, M. Ichiyanagi, N. Uchida, T. Iwasaki, H. Katao, K. Goto, S. Matsumoto, N. Hirata, S. Nakao, K. Uehira, M. Shinohara, H. Yakiwara, N. Kame, T. Urabe, N. Matsuwo, T. Yama, Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations, Earth Planets Space, Vol.58, No.12, 1599-1604, 2006.01.
87. Shimizu, H., Introduction to the special section for the 2005 West Off Fukuoka Prefecture Earthquake, Earth Planets Space, Vol.58, No.12, 3-5, 2006.01.
88. Hiroshi Shimizu, Takeshi Matsushima, Satoshi Matsumoto, Kenji Uehira, Norimichi Matsuwo, Atsushi Watanabe, Kazunari Uchida, Yuhki Kohno, Masaki Saito, Mio Hori, Hiroaki Takahashi, Masayoshi Ichiyanagi, Tomomi Okada, Naoki Uchida, Toshihiko Kanazawa, Masanao Shinohara, Takaya Iwasaki, Naoshi Hirata, Taku Urabe, Tomoaki Yamada, Kazuo Nakahigashi, Shin’ichi Hashimoto, Takeo Yagi, Yoshihisa Iio, Hiroshi Katao, Bogdan Enescu, Tomotake Ueno, Hiroki Miyamachi, Kazuhiko Goto, Shigeru Nakao, Hiroshi Yakiwara, Syuichiro Hirano, Nobuki Kame, Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations, earth, planets and space, 10.1186/BF03352668, 58, 12, 1599-1604, 2006.01, [URL], On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2-16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault..
89. Hiroshi Shimizu, Introduction to the special section for the 2005 West Off Fukuoka Prefecture Earthquake, earth, planets and space, 10.1186/BF03352656, 58, 12, 2006.01, [URL].
90. Hiroshi Shimizu, Takeshi Matsushima, Satoshi Matsumoto, Kenji Uehira, Norimichi Matsuwo, Atsushi Watanabe, Kazunari Uchida, Yuhki Kohno, Masaki Saito, Mio Hori, Hiroaki Takahashi, Masayoshi Ichiyanagi, Tomomi Okada, Naoki Uchida, Toshihiko Kanazawa, Masanao Shinohara, Takaya Iwasaki, Naoshi Hirata, Taku Urabe, Tomoaki Yamada, Kazuo Nakahigashi, Shin’ichi Hashimoto, Takeo Yagi, Yoshihisa Iio, Hiroshi Katao, Bogdan Enescu, Tomotake Ueno, Hiroki Miyamachi, Kazuhiko Goto, Shigeru Nakao, Hiroshi Yakiwara, Syuichiro Hirano, Nobuki Kame, Aftershock seismicity and fault structure of the 2005 West Off Fukuoka Prefecture Earthquake (MJMA7.0) derived from urgent joint observations, earth, planets and space, 10.1186/BF03352668, 58, 12, 1599-1604, 2006.01, [URL], On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2-16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault..
91. Kenji Uehira, Tomoaki Yamada, Masanao Shinohara, Kazuo Nakahigashi, Hiroki Miyamachi, Yoshihisa Iio, Tomomi Okada, Hiroaki Takahashi, Norimichi Matsuwo, Kazunari Uchida, Toshihiko Kanazawa, Hiroshi Shimizu, Precise aftershock distribution of the 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network, earth, planets and space, 10.1186/BF03352669, 58, 12, 1605-1610, 2006.01, [URL], The 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1-16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward..
92. Hiroshi Shimizu, Introduction to the special section for the 2005 West Off Fukuoka Prefecture Earthquake, earth, planets and space, 10.1186/BF03352656, 58, 12, 2006.01, [URL].
93. Atsushi Watanabe, Satoshi Matsumoto, Takeshi Matsushima, Kenji Uehira, Norimichi Matsuwo, Hiroshi Shimizu, Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake, earth, planets and space, 10.1186/BF03352674, 58, 12, 1633-1636, 2006.01, [URL], Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected..
94. Kenji Uehira, Tomoaki Yamada, Masanao Shinohara, Kazuo Nakahigashi, Hiroki Miyamachi, Yoshihisa Iio, Tomomi Okada, Hiroaki Takahashi, Norimichi Matsuwo, Kazunari Uchida, Toshihiko Kanazawa, Hiroshi Shimizu, Precise aftershock distribution of the 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network, earth, planets and space, 10.1186/BF03352669, 58, 12, 1605-1610, 2006.01, [URL], The 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1-16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward..
95. Atsushi Watanabe, Satoshi Matsumoto, Takeshi Matsushima, Kenji Uehira, Norimichi Matsuwo, Hiroshi Shimizu, Shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake, earth, planets and space, 10.1186/BF03352674, 58, 12, 1633-1636, 2006.01, [URL], Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefecture earthquake. Almost all of the LSPDs are oriented in the E-W direction, which is consistent with the maximum horizontal compressional axis inferred from the mechanism of the main shock. These E-to W-oriented LSPDs are caused by the alignment of stress-induced microcracks. Crack densities at most stations are estimated to be 0.02. Little spacial stress variation around focal region is suspected..
96. Yoshihisa Iio, Hiroshi Katao, Tomotake Ueno, Bogdan Enescu, Norio Hirano, Tomomi Okada, Naoki Uchida, Satoshi Matsumoto, Takeshi Matsushima, Kenji Uehira, Hiroshi Shimizu, Spatial distribution of static stress drops for aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, earth, planets and space, 10.1186/BF03352670, 58, 12, 1611-1615, 2006.01, [URL], We investigated the spatial distribution of static stress drops of the aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, with the aim of assessing the possibility that another earthquake will occur on the SE extension of the earthquake fault. The waveforms from six temporary online telemetry stations installed in and around the aftershock region were measured. Small stress drops were estimated for the aftershocks that occurred relatively distant from the SE and NE ends of the earthquake fault. Conversely, the aftershocks that occurred around the SE end of aftershock region are characterized by large stress drops. These results imply the possibility of a stress concentration at the SE edge of the main shock fault..
97. M. Hori, S. Matsumoto, K. Uehira, T. Okada, T. Yamada, Y. Iio, M. Shinohara, H. Miyamachi, H. Takahashi, K. Nakahigashi, A. Watanabe, T. Matsushima, N. Matsuwo, T. Kanazawa, H. Shimizu, Three-dimensional seismic velocity structure as determined by double-difference tomography in and around the focal area of the 2005 West off Fukuoka Prefecture earthquake, earth, planets and space, 10.1186/BF03352672, 58, 12, 1621-1626, 2006.01, [URL], On March 20, 2005 the West off Fukuoka Prefecture earthquake (magnitude of 7.0 on the JMA scale) occurred in southeastern Japan. The earthquake fault was a left-lateral strike-slip having a nearly vertical fault plane and a strike in the WNW-ESE direction. The largest aftershock with a magnitude of 5.8 (JMA) followed 1 month later. To gain more detailed aftershock data, several teams from different Japanese universities jointly installed a number of temporary seismic stations and positioned Ocean Bottom Seismometers (OBSs) immediately above the focal area. Double-difference tomography was used to estimate the three-dimensional (3D) (Zhang and Thurber, 2003) velocity structures in and around the focal area based on the travel time data collected during seismic observations. The high-velocity regions estimated by the inversion are located on the edge of the aftershock area and on the shallow part of asperity, as inferred from the slip distribution. Conversely, the Vp/Vs ratio is not always as high as that found at the location of the asperity. This finding suggests that the construction of the medium is not uniformly elastic but complex, with different relations between elastic constants and strength..
98. 井口正人・山本圭吾・橋本武志・筒井智樹・田中 聡・鬼澤真也・青木陽介・渡辺俊樹・大倉敬宏・清水 洋・八木原寛・宮町宏樹・平松秀行・為栗 健・高山鉄朗・福嶋麻沙代・Hetty TRIAST, 口永良部島火山における人工地震探査 —探査の概要と初動の走時について—, 東京大学地震研究所彙報, Vol.80, No.1, 11-40, 2005.12.
99. 井口正人・山本圭吾・橋本武志・筒井智樹・田中 聡・鬼澤真也・青木陽介・渡辺俊樹・大倉敬宏・清水 洋・八木原寛・宮町宏樹・平松秀行・為栗 健・高山鉄朗・福嶋麻沙代・Hetty TRIAST, 口永良部島火山における人工地震探査 —探査の概要と初動の走時について—, 京都大学防災研究所年報, No.48 B, 297-322, 2005.11.
100. 清水 洋, 速報福岡県西方沖の地震—緊急観測と地震活動の概要—, 月刊地震レポート サイスモ, 9巻,5号,14-15, 2005.05.
101. 山田知朗・篠原雅尚・金沢敏彦・平田 直・金田義行・高波鉄夫・三ヶ田 均・末広 潔・酒井慎一・渡辺智毅・植平賢司・村井芳夫・高橋成実・西野 実・望月公広・佐藤 壮・荒木英一, 稠密海底地震観測による2003年十勝沖地震の余震分布, 地震, 第2輯,57,3,281-290, 2005.02.
102. Yuhki Kohno, Takeshi Matsushima, Hiroshi Shimizu, The pressure sources beneath Unzen Volcano inferred from geodesic data, Science Reports of the Kyushu University, Department of Earth and Planetary Sciences, 22, 1, 29-39, 2005.02, We inferred the pressure sources under Unzen Volcano on Shimabara Peninsula, which erupted during 1990-1995 by using ground deformation data. It has been proposed that there are three pressure sources associated with the 1990-1995 activity beneath Unzen Volcano. But this model can not explain the ground deformation that was observed after the eruption stopped in 1995, because the upheaval was observed in the southwestern part of Shimabara Peninsula. We used 1991-2001 leveling data for searching. And we recalculated the source's parameters (horizontal location, depth, and volume change) by fitting. We modeled using Mogi Model. It was one of the most different points from previous studies that we modeled pressure sources considering the altitude of each observation station. As a result, we could explain the ground deformation around Unzen Volcano by four-source model..
103. 河野裕希・松島 健・清水 洋, 測地学データから推定された雲仙岳下における圧力源, 九州大学大学院理学研究院研究報告(地球惑星科学), Vol.22,No.1,29-39, 2005.01.
104. Okubo, A., Y. Tanaka, M. Utsugi, N. Kitada, H. Shimizu, and T. Matsushima, Magnetization intensity mapping on Unzen Volcano, Japan, determined from high-resolution, low-altitude helicopter-borne aeromagnetic survey, Earth, Planets and Space, 57, 8, 743-753, Vol.57, No.8, 743-753, 2005.01.
105. 中田節也,佐久間澄夫,宇都浩三,清水 洋, 雲仙火道掘削の科学的成果の概要, 地熱技術, 30巻,1/2号,45-52, 2005.01.
106. 清水 洋, 雲仙火山活動, 日本応用地質学会九州支部会報「GET九州」, No.26, 35-39, 2005.01.
107. 福岡県西方沖地震・大学合同地震観測班・清水 洋, 緊急地震観測による福岡県西方沖地震の特徴について, 九州大学西部地区自然災害資料センターニュース, No.33,3-6, 2005.01.
108. 植平賢司・清水 洋, 福岡県西方沖地震の驚き・教訓, 科学, Vol.75,795-797, 2005.01.
109. Ayako Okubo, Yoshikazu Tanaka, Mitsuru Utsugi, Naoto Kitada, Hiroshi Shimizu, Takeshi Matsushima, Magnetization intensity mapping on Unzen Volcano, Japan, determined from high-resolution, low-altitude helicopter-borne aeromagnetic survey, earth, planets and space, 10.1186/BF03351853, 57, 8, 743-753, 2005.01, [URL], On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-bome aeromagnetic survey at two flight altitudes, using spiral trajectories for the first time, over Unzen Volcano in the framework of the Unzen Scientific Drilling Project (USDP). This study obtained more detailed and new information than the previous aeromagnetic studies in Unzen volcano about the geological features, for understanding the history and eruption mechanism of the Unzen volcano. Therefore, we conducted a magnetization intensity mapping on the volcano, on the assumption that the magnetic anomalies are caused by the terrain magnetized in the same direction as the present Earth's magnetic field and the magnetization intensity varies only laterally. This map shows good agreement with the geologic features, especially the hydrothermal alteration zone and the collapsed pyroclastic deposits. In addition, even in the area covered by lavas, the magnetization intensities show various values corresponding to each eruption event. It may be considered that the differences in magnetic properties reflect different oxygen fugacity in rocks during their cooling time period. Local magnetization lows on Heisei-Shinzan suggest that the Heisei lava produced by the 1991-1995 eruption has not yet been cooled enough..
110. 植平賢司・松島 健・清水 洋・安達繁樹・齊藤幸賢, 訂正 衛星電話を利用した地震観測システムの開発 ―無人島における定常地震観測の実現―, 地震2, 57, 70, 2004.08.
111. 筒井智樹・松島 健・清水 洋, 雲仙火山の浅部構造に対する擬似反射記録法の適用の試み, 火山, Vol.49,No.3,143-151, 2004.06.
112. Shinohara, M., T. Yamada, T. Kanazawa, N. Hirata, Y. Kaneda, T. Takanami, H. Mikada, K. Suyehiro, S. Sakai, T. Watanabe, K. Uehira, Y. Murai, N. Takahashi, N. Nishino, K. Mochizuki, T. Sato, E.Araki, R. Hino, K. Uhira, H. Shiobara, and H. Shimizu, Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network, Earth Planets Space, 56, 3, 295-300, Vol.56, No. 3, 2004.01.
113. Yamawaki, T., S. Tanaka, S. Ueki, H. Hamaguchi, H. Nakamichi, T. Nishimura, J. Oikawa, T. Tsutsui, K. Nishi, H. Shimizu, S. Yamaguchi, H. Miyamachi, H. Yamasato, Y. Hayashi, Three-dimensional P-wave velocity structure of Bandai volcano in northeastern Japan inferred from active seismic survey, J. Volcanol. Geotherm. Res., 10.1016/j.jvolgeores.2004.07.010, 138, 3-4, 267-282, Vol.138,267-282, 2004.01.
114. 大久保綾子・田中良和・宇津木 充・北田直人・清水 洋・松島 健, 雲仙火山における低高度な高密度空中磁気探査, 京都大学防災研究所年報, 第47号B,735-742, 2004.01.
115. 大島弘光・青山 裕・筒井智樹・西村太志・田中 聡・藤田英輔・熊谷博之・中道治久・及川 純・大湊隆雄・小林知勝・武尾 実・鬼沢真也・高木憲朗・中野 優・井口正人・為栗 健・, 日本の火山性地震・微動のデータベース, 日本の火山性地震・微動のデータベース, 649, 2004.01.
116. Shinohara, M., T. Yamada, T. Kanazawa, N. Hirata, Y. Kaneda, T. Takanami, H. Mikada, K. Suyehiro, S. Sakai, T. Watanabe, K. Uehira, Y. Murai, N. Takahashi, M. Nishino, K. Mochizuki, T. Sato, E. Araki, R. Hino, K. Uhira, H. Shiobara, and H. Shimizu, Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network, Earth Planets Space, 56, 3, 295-300, Vol.56, 295-300, 2004.01.
117. Masanao Shinohara, Tomoaki Yamada, Toshihiko Kanazawa, Naoshi Hirata, Yoshiyuki Kaneda, Tetsuo Takanami, Hitoshi Mikada, Kiyoshi Suyehiro, Shin'ichi Sakai, Tomoki Watanabe, Kenji Uehira, Yoshio Murai, Narumi Takahashi, Minoru Nishino, Kimihiro Mochizuki, Takeshi Sato, Ei'ichiro Araki, Ryota Hino, Kouichi Uhira, Hajime Shiobara, Hiroshi Shimizu, Aftershock observation of the 2003 Tokachi-oki earthquake by using dense ocean bottom seismometer network, earth, planets and space, 10.1186/BF03353054, 56, 3, 295-300, 2004.01, [URL], The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We started the OBS observation four days after the mainshock for an observation period of approximately two months. In the middle of the observation period, nine OBSs near the epicenter of the mainshock were recovered to clarify the depth distribution of aftershocks near the mainshock. From the data overall OBS, seventy-four aftershocks were located with high spatial resolution. Most of the aftershocks were located in a depth range of 15-20 km and occurred within the subducting oceanic crust, the 5.5-km/s layer of the landward plate and the plate boundary. No aftershocks were found in the mantle of the subducting plate. The low seismic activity beneath the trench area where the water depth is greater than about 2000 m suggests a weak coupling between the two plates. The depth of the mainshock is inferred to be 15-20 km from the aftershock distribution..
118. 松島 健・清水 洋・植平賢司・松尾のり道, 雲仙岳の深部マグマ溜まり, 月刊地球, Vol.25, 47-50, 2003.01.
119. The Volcanological Society of Japan, and National Committee for Volcanology and Chemistry of the Earth's Interior, Science Council of Japan, Reports on Volcanic Activities and Volcanological Studies in Japan, for the Period from 1999-2002, Reports on Volcanic Activities and Volcanological Studies in Japan, for the Period from 1999-2002, 94pp., 2003.01.
120. Nakada, S., Y. Miyabuchi, K. Watanabe, Y. Sudo, H. Hoshizumi, K. Uto, A. Matsumoto, and H. Shimizu, Unzen and Aso Volcanoes, IUGG 2003 Sapporo,Japan, Field Trip Guidebook, A3:1-A3:38, 2003.01.
121. Matthews, J. P., H. Kamata, S. Okuyama, Y. Yusa, and H. Shimizu, Surface height adjustments inpyroclastic-flow deposits observed at Unzen volcano by JERS-1 SAR interferometry, J.Volcanol Geotherm. Res., 10.1016/S0377-0273(03)00112-4, 125, 3-4, 247-270, Vol.125, 247-270, 2003.01.
122. 大久保綾子・田中良和・北田直人・宇津木充・清水 洋・松島 健, 雲仙火山における空中磁気測量について, 京都大学防災研究所年報, 第46号B,739-747, 2003.01.
123. 鬼澤真也・大島弘光・青山 裕・森  済・前川徳光・鈴木敦生・岡田 弘・筒井智樹・松尾のり道・及川 純・大湊隆雄・山本圭吾・森 健彦・平 貴昭・宮町宏樹・小山順ニ・蓬田 清, 有珠火山における人工地震探査−−観測および初動の読み取り−−, 東京大学地震研究所彙報, 第78号,2冊,121-143, 2003.01.
124. 清水 洋, マグマ供給系の構造と噴火機構のモデル化 ‐地球物理観測からのアプローチ‐, 火山, Vol.48, No.1, 151-156, 2003.01.
125. M. Nakada, M. Tahara, H. Shimizu, S. Nagaoka, K. Uehira and S. Suzuki, Late Pleistocene crustal uplift and gravity anomaly in eastern part of Kyushu, Japan, and its geophysical implications, Tectonophisics, 10.1016/S0040-1951(02)00161-0, 351, 4, 263-283, Vol.351, 263-283, 2002.07.
126. 高橋成美・三ヶ田均・末広潔・卜部卓・清水洋, エアガン人工地震データ解析による島原半島の地殻構造, 火山, Vol.47, 461-473, 2002.01.
127. 馬越孝道・清水洋・松尾のり道, 雲仙普賢岳溶岩ドーム成長にともなう地震活動(1993年11月〜1994年1月)ー波形の相互相関による地震のグループ分けー, 火山, Vol.47, 43-55, 2002.01.
128. 榎田充哉・福田睦寿・清水 洋・福井理作・市川仁士・岸原信義, 水文諸量の時系列解析手法に関する検討(2)−2変量時系列解析手法の提案−, 水文・水資源学会誌, Vol.15,23-38, 2002.01.
129. 須藤靖明・筒井智樹・小野博尉・田中良和・橋本武志・外 輝明・増田秀晴・迫 幹雄・吉川 慎・坂中伸也・森 健彦・中坊 真・長谷英彰・松下誠司・筒井麻貴・吉川美由紀・岡田 弘, 阿蘇火山における人工地震探査−観測および初動の読みとり−, 東京大学地震研究所彙報, Vol.77,303-336, 2002.01.
130. 鈴木貞臣・サデギ ホセイン・竹中博士・清水 洋・JUN Myung-soon, 地震トモグラフィー及び韓国データによる九州・琉球島弧の背弧深部構造, フィリピン海スラブの沈み込みと島弧・背弧の地球物理・京都大学防災研究所研究集会(一般)13K-7報告書, 223-230, 2002.01.
131. 中村めぐみ・松本 聡・植平賢司・清水 洋, 稍深発地震に見られる変換波を用いた九州地方及びその周辺におけるモホ面の推定, フィリピン海スラブの沈み込みと島弧・背弧の地球物理・京都大学防災研究所研究集会(一般)13K-7報告書, 246-255, 2002.01.
132. 清水 洋・植平賢司・松本 聡・松島 健・松尾のり道, 布田川-日奈久断層系における地震活動, 月刊地球, Vol.38, 128-133, 2002.01.
133. 清水 洋・松本 聡・植平賢司・松尾のり道・大西正純, 雲仙火山における火道探査実験, 月刊地球, Vol.24, 878-882, 2002.01.
134. 植平賢司・清水 洋・松尾のり道・後藤和彦, 四国・中国西端から九州にかけての深発地震面の形状と発震機構, 月刊地球, Vol.23, No.10, 669-673, 2001.01.
135. K. Umakoshi, H. Shimizu, N. Matsuwo, Volcano-tectonic seismicity at Unzen volcano, Japan, 1985-1999, Journal of Volcanology and Geothermal Research, 10.1016/S0377-0273(01)00238-4, 112, 1-4, 117-131, 2001.12, [URL], The eruption of Unzen Volcano in southwestern Japan began on November 17, 1990, and extrusion of a dacite lava dome lasted from May 20, 1991, until early February 1995. The seismic network operated by the Shimabara Earthquake and Volcano Observatory of Kyushu University (SEVO) located about 4000 volcano-tectonic earthquakes during the period from 1985 to 1999. The seismicity was greatest in the period between one year before the eruption and the start of the lava dome extrusion. The hypocenters were mostly distributed within a 12 × 20 km area on the western side of the volcano. An inclined seismic-aseismic boundary shallows eastward to the summit, probabaly reflecting the thermal structure beneath the volcano. Within 4 km west from the summit, however, the seismicity is very low. It was suggested that the high-temperature, ductile and low-Q bodies exist there. We obtained focal mechanism solutions for about 90 earthquakes. Among them, orientations of P-axes for the earthquakes occurring on the upper side of the inclined seismic-aseismic boundary suggest a pressure source located below this boundary. Although most of the focal mechanisms have T-axes nearly horizontal in the N-S direction, consistent with the N-S extensional tectonic stress, several earthquakes whose focal mechanisms are inconsistent with the tectonic stress were observed within a horizontal distance of 5 km from the summit. This is because the pressurization of volcanic gas or magma beneath the summit produced a strong local stress in the summit surroundings..
136. 松島 健・清水 洋・植平賢司・松尾のり道, 水準測量・GPS測量から判明した雲仙火山の再膨張について, 月刊地球, Vol.23, No.8, 540-544, 2001.01.
137. 植平賢司,松島 健,清水 洋,安達繁樹,齊藤幸賢, 衛星電話を利用した地震観測システムの開発 —無人島における定常地震観測の実現—, 地震2, Vol.53, No.2, 181-184, 2000.12.
138. M. J. Wooster, T. Kaneko, S. Nakada, H. Shimizu, Discrimination of lava dome activity styles using satellite-derived thermal structures, Journal of Volcanology and Geothermal Research, 10.1016/S0377-0273(00)00183-9, 102, 1-2, 97-118, 2000.10, [URL], Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the remote sensing data itself, whilst using any ancillary field data to guide analysis and the necessary assumptions. This study investigates a method by which such discrimination maybe accomplished for activity at lava domes. For this purpose we use three Landsat Thematic Mapper (TM) scenes of Unzen Volcano (Japan) to determine the temperature structure of the active lava dome existing during 1991-1993. These data are of particularly high quality since the TM scenes were obtained during night-time overpasses and a large amount of supplementary data are available to parameterise the model used to retrieve the subpixel temperature structure of the dome surface. The results are matched to near-contemporaneous geological sketch maps of the then current situation in order to identify the TM pixel groupings corresponding to the different styles of dome activity, namely fumarolic degassing, exogenous and endogenous dome growth, and collapse deposits resulting from block and ash flows, small scale pyroclastic flows and rockfalls. The spatial and statistical characteristics of the resultant TM-derived temperature distributions are then investigated to determine rule-based criteria that may be used to differentiate the activity styles of lava domes based on their thermal structure within the remote sensing data. Results indicate that fumarolically active locations, regions of active dome growth, and areas of collapse deposition on the Unzen dome can be differentiated using only the statistical distribution of the hotspot temperatures and fractional areas identified via analysis of TM imagery. The statistics derived for hotspots at the fumarolically active areas show them to be, in general, significantly hotter (and smaller) than those found at the areas of active dome growth, whereas hotspots retrieved on the areas of collapse deposition are cooler (and larger). We find that the frequency distributions of hotspot temperatures extracted for the areas of endogenous and exogenous dome growth exhibit statistically inseparable means, but that discrimination between these two dome growth styles is possible using analysis of the spatial arrangement of the retrieved hotspots. Specifically the exogenous lobes are characterised by structures interpreted to be the effusing vent of new magma and the collapsing lobe front. These features are absent at locations showing only endogenous growth. It is hoped that the criteria developed here will prove useful during future quantitative analysis of the extended TM time-series available for Unzen, and for identification of thermal anomalies of uncertain origin at other active domes observed via satellite remote sensing..
139. Ken'Ichiro Yamashina, Hiroshi Shimizu, Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan, Journal of Volcanology and Geothermal Research, 10.1016/S0377-0273(98)00121-8, 89, 1-4, 43-55, 1999.04, [URL], Tilt observations at FG1 station located about 680 m west of the central crater of Unzen volcano, southwestern Japan, revealed details about the process of magma ascent during the mid-May 1991 crisis, just before the appearance of a lava dome. Crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike, modeled by a vertical line source and a planer tensile dislocation in an elastic half space. Marked change in tilt was observed at first in the EW direction at FG1 during May 11-14, suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater. After the interruption of the rise of the magma column, magma began to intrude laterally as a dike on May 15 toward approximately the N80°W direction. This resulted in a large tilt change essentially in the NS component at FG1. The horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m. On May 17, the magma column started to rise again, probably because the impediment to magma ascent was fractured by the growth of the dike. The top of the magma column finally reached to the crater bottom probably on May 19, and a new lava dome was actually observed on May 20. After which, the remarkable upward tilt to the south at FG1 due to the dike intrusion decreased rapidly. The deformation after May 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of May 1991. Although large errors are inevitable in estimated values because of a lack of sufficient data, the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions..
140. Setsuya Nakada, Hiroshi Shimizu, Kazuya Ohta, Overview of the 1990-1995 eruption at Unzen Volcano, Journal of Volcanology and Geothermal Research, 10.1016/S0377-0273(98)00118-8, 89, 1-4, 1-22, 1999.04, [URL], Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1 x 108 m3 (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230-540 m high). The eruption finished with extrusion of a spin endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO2 flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995..
141. Setsuya Nakada, Yoshinobu Motomura, Hiroshi Shimizu, Manner of magma ascent at Unzen Volcano (Japan), Geophysical Research Letters, 10.1029/95GL00002, 22, 5, 567-570, 1995.01, [URL], Juvenile materials were found among products of phreatomagmatic eruptions that preceded dacite dome growth at Unzen Volcano in 1991. They give evidence showing that the hydrous magma started degassing with the resultant crystallization around 100 MPa, and was quenched soon thereafter. Ascending at a rate as low as 13 m/d while degassing, however, the still‐molten part inside reacted with water. Phreatomagmatic eruptions started when the magma reached about 1.2 km in depth, and strong ones started at about 0.6 km. Volcanic tremors had occurred at these depths, where the sources of vulcanian explosions and the ground deformation were also located, implying the existence of a possible gas pocket..
142. Hiroshi Shimizu, Sadato Ueki, Junji Koyama, A tensile - shear crack model for the mechanism of volcanic earthquakes, Tectonophysics, 10.1016/0040-1951(87)90023-0, 144, 1-3, 287-300, 1987.12, [URL], Temporary seismic observations were carried out for about 1 month at Miyakejima Island on the Izu-Bonin arc after an eruption on October 3, 1983. More than 3000 seismic events were observed and they are classified into 4 different categories: short-period earthquakes, long-period earthquakes, isolated tremors and continuous tremors. Two active source regions of the short-period earthquakes were located along the old caldera rim of the Miyakejima volcano. One region was very close to the eruptive fissures. Another was about 1-2 km northwest of the fissures. Almost all the earthquakes in the former region showed dilatant first motions of P-waves at all the stations in the island. On the other hand, almost all events in the latter region showed compressive first motions of P-waves at all the stations. The first motions of P-waves for these earthquakes cannot be explained by the quadrant-type pattern due to a double-couple force system. Nucleation of a tensile crack coupled with a shear crack (a tensile-shear crack model) is proposed as a source mechanism of the short-period earthquakes. Moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed P- and S-wave data. Results suggest that tensile cracking (opening or closing) is dominant for generating those earthquakes in comparison with shear cracking. The strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures. The earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile-shear cracks due to the excess pressure of intrusive magma. In contrast, the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile-shear cracks, and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption. Moment tensor analysis of the tensile-shear crack model in this study demonstrates the non double-couple source mechanism for volcanic earthquakes related to magmatic activity..
143. Hiroshi Shimizu, Sadato Ueki, Junji Koyama, A tensile - shear crack model for the mechanism of volcanic earthquakes, Tectonophysics, 10.1016/0040-1951(87)90023-0, 144, 1-3, 287-300, 1987.12, [URL], Temporary seismic observations were carried out for about 1 month at Miyakejima Island on the Izu-Bonin arc after an eruption on October 3, 1983. More than 3000 seismic events were observed and they are classified into 4 different categories: short-period earthquakes, long-period earthquakes, isolated tremors and continuous tremors. Two active source regions of the short-period earthquakes were located along the old caldera rim of the Miyakejima volcano. One region was very close to the eruptive fissures. Another was about 1-2 km northwest of the fissures. Almost all the earthquakes in the former region showed dilatant first motions of P-waves at all the stations in the island. On the other hand, almost all events in the latter region showed compressive first motions of P-waves at all the stations. The first motions of P-waves for these earthquakes cannot be explained by the quadrant-type pattern due to a double-couple force system. Nucleation of a tensile crack coupled with a shear crack (a tensile-shear crack model) is proposed as a source mechanism of the short-period earthquakes. Moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed P- and S-wave data. Results suggest that tensile cracking (opening or closing) is dominant for generating those earthquakes in comparison with shear cracking. The strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures. The earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile-shear cracks due to the excess pressure of intrusive magma. In contrast, the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile-shear cracks, and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption. Moment tensor analysis of the tensile-shear crack model in this study demonstrates the non double-couple source mechanism for volcanic earthquakes related to magmatic activity..
144. Azusa Nishizawa, Kiyoshi Suyehiro, Hiroshi Shimizu, Seismic refraction experiment at the amami plateau, journal of physics of the earth, 10.4294/jpe1952.31.159, 31, 3, 159-171, 1983.01, [URL], Seismic refraction profiles were shot at the Amami Plateau, an anomalous bathymetric high among many in the West Philippine Basin. A model of the crustal structure beneath the Amami Plateau consisting of materials with compressional wavespeeds 2.0, 2.8-3.8, 5.4-5.6, and 6.7 km/sec, with thicknesses 0.3-0.7, 1.0-2.0, and 3.0-4.0 km, layered from the top, was obtained. This indicates that the 5.4-5.6 km/sec layer characterizes the structure of the plateau and implies that a wavespeed of less than 5.6 km/sec occupies more than half the crust. It is suggested, therefore, that the crustal structure is different from that of normal oceanic. The sedimentary shear wavespeed was found to be 0.39 km/sec and Poisson's ratio 0.48 from P-PS delay time produced by phase conversion at the base of the sediment..
145. Azusa Nishizawa, Kiyoshi Suyehiro, Hiroshi Shimizu, Seismic refraction experiment at the amami plateau, journal of physics of the earth, 10.4294/jpe1952.31.159, 31, 3, 159-171, 1983.01, [URL], Seismic refraction profiles were shot at the Amami Plateau, an anomalous bathymetric high among many in the West Philippine Basin. A model of the crustal structure beneath the Amami Plateau consisting of materials with compressional wavespeeds 2.0, 2.8-3.8, 5.4-5.6, and 6.7 km/sec, with thicknesses 0.3-0.7, 1.0-2.0, and 3.0-4.0 km, layered from the top, was obtained. This indicates that the 5.4-5.6 km/sec layer characterizes the structure of the plateau and implies that a wavespeed of less than 5.6 km/sec occupies more than half the crust. It is suggested, therefore, that the crustal structure is different from that of normal oceanic. The sedimentary shear wavespeed was found to be 0.39 km/sec and Poisson's ratio 0.48 from P-PS delay time produced by phase conversion at the base of the sediment..
146. Kiyoshi Suyehiro, Azusa Nishizawa, Hiroshi Shimizu, Microearthquake Activity in the West Philippine basin and Northern Ryukyu Trench Region as Observed by Ocean Bottom Seismometers at the Amami Plateau, journal of physics of the earth, 10.4294/jpe1952.30.509, 30, 6, 509-516, 1982.01, [URL].
147. 田中良和・橋本武志・北田直人・大久保綾子・宇津木充・清水 洋・松島 健, 雲仙火山における空中磁気測量について, 平成14年度雲仙火山科学掘削成果報告書.
148. Kiyoshi Suyehiro, Azusa Nishizawa, Hiroshi Shimizu, Microearthquake Activity in the West Philippine basin and Northern Ryukyu Trench Region as Observed by Ocean Bottom Seismometers at the Amami Plateau, journal of physics of the earth, 10.4294/jpe1952.30.509, 30, 6, 509-516, 1982.01, [URL].

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