Reports

Papers

1.	Nakao, S., Morita, Y., Yakiwara, H., Oikawa, J., Ueda, H., Takahashi, H., Ohta, Y., Matsushima, T., Iguchi, M., Volume change of the magma reservoir relating to the 2011 Kirishima Shinmoe-dake eruption-Charging, discharging and recharging process inferred from GPS measurements, Earth Planets Space, doi:10.5047/eps.2013.05.017, 65, 6, 505-515, 2013.05, Using GPS data, we evaluate the volume change of the magma reservoir associated with the eruption of Kirishima Shinmoe-dake volcano, southern Kyushu, Japan, in 2011. Because ground deformation around Shinmoe-dake volcano is strongly affected not only by regional tectonic movement but also by inflation of Sakurajima volcano located approximately 30-40 km to the southwest, we first eliminate these unwanted contributions from the observed data to extract the signals from Shinmoe-dake volcano. Then, we estimate the source locations and volume change before, during, and after the highest eruptive activity occurring between January 26 and 31. Our model shows that the magma began to accumulate about one year prior to the sub-Plinian eruption, with approximately 65% of the accumulated magma being discharged during the peak of the eruptive activity, and that magma accumulation continued until the end of November 2011. An error analysis shows that the sources during the three periods indicated above are located in almost the same position: 5 km to the northwest of the summit at a depth of 8 km. The 95% confidence interval of the estimated source depth is from 7.5 to 13.7 km. .
2.	Matsumoto, S., Shimizu, H., Matsushima, T., Uehira, K., Yamashita, Y., Nakamoto, M., Miyazaki, M., Chikura, H., 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, 2013.05, 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. .
3.	Keigo Yamamoto,, Tadaomi Yamamoto, Tetsuro Takayama, Nobuo Ichikawa, Takahiro Ohkura, Shin Yoshikawa, Hiroyuki Inoue,, TAKESHI MATSUSHIMA, Kazunari Uchida, Manami Nakamoto, Vertical Ground Deformation Associated with the Volcanic Activity of Sakurajima Volcano, Japan during 1996-2010 as Revealed by Repeated Precise Leveling Surveys, Bull. Volcanol. Soc. Japan, 58, 1, 137-151, 2013.04.
4.	Estimation of subsurface structure using microtremor H/V spectral ratio in the Shimabara peninsula.
5.	Relation between Chijiwa Caldera and Activity of Unzen Volcano Matsushima, T. and Y. Kohno Earth Monthly, Vol.48, No.2, 122-127, 2006.
6.	Observation tests of the Antarctic penetrator on the Mizuho Plateau in JARE-43 summer operation Matsushima, T., M. Yamashita, T. Yasuhara, K. Horiguchi, H. Miyamachi, S. Toda, M. Takeda, A. Watanabe, and K. Shibuya Antarctic Record, Vol.47, No.3, 395-408, 2003.
7.	Matsushima, T. and A. Takagi, GPS and EDM Monitoring of Unzen Volcano Ground Deformation, Earth, Planets and Space, 52, 11, 1015-1018, Vol.52, 1015-1018, 2000.01.
8.	Volcanoes in Northern Mariana Islands Takeshi MASUSHIMA and Teruyuki KATO(1999) Bulletin of the Volcanological Society of Japan,Vol.44, No.3,179-182.

Presentations

1.	Takeshi MATSUSHIMA, Kaori MORITA, Yuki KOGA, Hiroshi SHIMIZU, Vertical ground deformation of Ioyama, Kirishima volcanoes measured by precise leveling survey (during June 2015 - May 2018), Cities on Volcanoes 10, 2018.09, Ioyama of Kirishima Volcanoes is located in Ebino Kogen volcanic area, southern Kyushu, Japan. In Ioyama, volcanic earthquakes and tremor have occurred since December 2013. Since December 2015, the fumarolic gas and the expansion of the thermal anomaly area are seen around the Ioyama area. In April 2018, Ioyama erupted for the first time in 250 years. We conducted the precise leveling survey in the Ebino Kogen volcanic area for 11 times from June 2015 to May 2018 in order to accurately measure the vertical deformation. We estimated pressure source models assuming the presence of an inflation spherical source as Mogi’s model. We obtained the optimum value of the expansion amount, the horizontal position and the depth of the pressure source by the grid search method. As a result, inflation of spherical source has been inferred 150 m east of Ioyama’s fumarolic gas area, the depth about 700 m from the surface. The lower limit of low resistivity layer assumed to be the clay layer is estimated in this depth (Aizawa et al., 2013). Accordingly, the estimated inflation source is located just under the impermeable clay layer, through the small crack of this clay layer, fumaroles and hot springs are jetting out to the ground. The increase of pressure source volume since June 2015 is up to 4.8x104 m3 in November 2016. In the leveling surveys between October 2017 and March 2018, a sudden uplift phenomenon up to 15.5 mm was observed. From April 2018, the fumarocious activity became very active and eruption on April 19th occurred. In this study, we found that the volume change of pressure source was fluctuating 2 or 3 month prior to the surface activity of Ioyama, and the leveling survey helps predict volcanic activities..
2.	Fukui, F., Matsushima, T., Oikawa, J., Watanabe, A., Okuda, T., Ozawa, T., Kohno, Y., Miyagi, Y., Crustal deformation of Miyakejima volcano, Japan since the eruption of 2000 using dense GPS campaign observation, 2013 Fall Meeting, AGU, V51E-2739, 2013.12, Miyakejima is an active volcanic Island located about 175 km south of Tokyo, Japan. Miyakejima volcano erupted approximately every 20 years in the past 100 years. The latest eruptive activities since 2000 was different from those of the last 100 years, in that the activities included a caldera formation for the first time in 2500 years and gigantic volcanic gas emission that forced islander to evacuate over four and half years. In 2000, a dense GPS observation campaign had detected the magma intrusion in detail (e.g., Irwan et al., 2003; Murase et al., 2006). However, this campaign observation ceased from 2002 to 2010 because a large amount of volcanic gas prevented from entering to the island. Since 2011, we restarted the campaign observation by the dense GPS network, and examined the ongoing magma accumulation process beneath Miyakejima volcano to get insights about the future activity. In this analysis, we combined the data of our campaign observations, the data of the University Union in 2000, and the GEONET data. The observation data were analyzed by RTK-LIB (Takasu et al., 2007) using GPS precise ephemeris from IGS. We estimated the locations and volumes of the pressure sources beneth Miyakejima using an elevation-modified Mogi model (Fukui et al., 2003) and open crack model (Okada, 1992) during the two periods (2000 ~ 2012 and 2011 ~ 2012). We used the software of Magnetic and Geodetic data Computer Analysis Program for Volcano (MaGCAP-V) (Fukui et al., 2010), and estimated the source parameters by trial and error. During 2000 and 2012, a contracting spherical source and contracting dyke were estimated beneath the caldera and at the southwestern part of the island, respectively. In contrast, during 2011 and 2012, an spherical inflation source was estimated a few km beneath the caldera. This result suggest that Miyakejima is now storing new magma for the next eruption. Geospatial Information Authority of Japan (GSI) (2011) suggested that the inflation started since 2006. We will also carry out the GPS observation this autmn, and will present the result during 2012-2013..
3.	Fukui, M., Matsushima, T., Yumitori, N., Oikawa, J., Watanabe, A, Okuda, T., Ozawa, T., Kohno, Y., Miyagi, Y., Crustal deformation of Miyakejima volcano, Japan since the eruption of 2000 using dense GPS campaign observation, IAVCEI 2013 Scientific Assembly, 1W_2F-P17, 2013.07.
4.	Shimizu, H., Matsushima, T., Matsumoto, S., Uehira, K., Fukui, R., Uchida, K., Umakoshi, K., Nakada, S., The 1990-1995 Eruption and Current Volcanic Activity in Unzen Volcanic Area Global Geopark, Japan, 5th International UNESCO Conference on Geoparks, 1-P-07, 2012.05, Unzen Volcanic Area Global Geopark is located at the western end of the central Kyushu rift valley, Southwest Japan. Unzen Volcano, the symbol of the geopark, is an active volcano born about 500 thousand years ago, and has repeatedly erupted. The last eruption began in 1990 and continued until 1995. Before the 1990-1995 eruption, the precursory activity of volcano-tectonic earthquakes was observed by the seismic network of Kyushu Univ. and Japan Meteorological Agency (JMA). The eruptive activity started as phreatic explosions first, and then changed to phreatomagmatic explosions. In May 1991, the lava dome appeared in the crater. We successfully detected the swarm of shallow volcanic earthquakes, swelling of volcanic edifice and rapid demagnetization beneath the crater. In the growing process of lava dome complex, the pyroclastic flows had frequently occurred by partial collapse of the lava dome. Forty four people (inhabitants, policemen and mass media people) were killed by the pyroclastic flows of 3 June 1991 and 23 June 1993. The various geophysical observations have been carried out during and after the 1990-1995 eruption. These observations enabled us to image the magma supplying system and to evaluate the current activity of the volcano. The seismicity in and around the Unzen volcanic area decreased after the eruption, and the low level seismicity has still continued. The observation of the magnetic total force shows that the gradual demagnetization had continued until 1999, but stopped in 2000. The temperature of fumarole at the lava dome has decreased monotonously since the lava effusion ceased in 1995. The geodetic measurements revealed that the pressure of shallow magma reservoirs (Source A and B) also decreased after the eruption. These suggest that a sequence of the last eruption has been completed. On the other hand, the magma had been still supplied to the deeper magma reservoirs (Source C and D) until about 2000. The magma accumulation at the deeper reservoirs has not been obviously detected after 2001, however, the preparation process for the future eruption has probably started beneath the Unzen volcanic area..
5.	Yamashita, Y., Matsushima, T., Matsumoto, S., Shimizu, H., Nakamoto, M., Miyazaki, M., Uehira, K., Geophysical Observation and Monitoring for Eruptive Activity of Shinmoe-dake, Kirishima Geopark, 5th International UNESCO Conference on Geoparks, 1-P-11, 2012.05, Shinmoe-dake volcano is located in the Kirishima volcanic group (Kirishima Geopark) in Kyushu, Japan. Major eruptions for Shinmoe-dake occurred in 1716 -1717: fall out deposits, pyroclastic flows and mudflows were widely dispersed around the volcano [Imura and Kobayashi (1991)]. Recently, on January 19th, 2011, Shinmoe-dake began a first magmatic eruption in about 300 years, in which eruption type changed to Vulcanian after three sub-Plinian events in January 26-27th, 2011, and volcanic activity is still continuing. For the volcanic disaster-prevention, it is very important for monitoring the volcanic activity to detect movement of magma from the chamber to the active crater in real time. However there were a few on-line observation stations (e.g., seismometer, tiltmeter, infrasound microphone) in the Kirishima area. Therefore, we installed two temporal on-line observation stations: Shinyu (KU.KRSY , 3km WSW from the crater) with a broadband seismometer and an infrasound microphone, Onami-ike Tozanguchi (KU.KRON, 4km WNW) with a broadband seismometer and a tiltmeter. These data has been transmitted to SEVO, Kyushu University using a mobile phone data terminal since January 28th, 2011. In addition, these data has been also transmitted to Japan Meteorological Agency for monitoring the volcanic activity. During observation, many explosive eruptions and volcanic tremors were occurred at the volcano. These events were recorded clearly by our observation network. From the end of January to February 2011, harmonic tremors were recorded several times by broadband seismometers and infrasonic microphone with almost similar waveform. The lag times of two waveforms were approximately 6-7 seconds. Considering the difference of velocity between P-wave and sonic wave, the source of harmonic tremor was in the very shallow part of volcano (just under the crater). It is generally difficult to detect the location of volcanic earthquake and tremor but information of these locations is very important because these events are believed to reflect the movement of magma. Our result suggests that the observation with combination of broadband seismometer and infrasound microphone is useful to detect the volcanic event occurring very shallow part of volcano and to assess the volcanic activity..
6.	Itoya, N., Matsushima, T., Estimation of Subsurface Structure in the Unzen Volcanic Area Using Microtremor H/V Spectral Ratio, 5th International UNESCO Conference on Geoparks, 3-P-09, 2012.05, Unzen Volcanic Area Geopark is located on the west edge of the Beppu-Shimabara graben which crosses the center part in Kyushu island from east to west. Seventy percent of the Geopark area is composed by volcanic product from the volcano. From the contour map of peak period for long-period ground motions in Japan, it has been estimated that the ground motions are strongly amplified in Unzen Volcanic Area Geopark. It is very important to study the mechanism of this phenomenon from the viewpoint of disaster prevention. In order to estimate ground structure in the Geopark area using microtremor H/V spectra (horizontal-to-vertical spectral ratio), we carried out microtremor observations at 60 sites throughout the area Using data from these observation sites, we traced a contour map of primary natural peak period. Peak periods of 5 - 6 s in the H/V spectra were observed at many of the observation sites to the east of the Shimabara Peninsula, where thick volcanic sediments are distributed. It is thought that the thick volcanic sediment layer is the cause of such long peak periods in the H/V spectra. In the central western area, there are no remarkable peaks in the observed H/V spectra. According to explosion seismic research, this area corresponds to a rock layer having Vp = 3.5 km/s; this is a solid lava layer that extends to the ground surface. This structure is reflected in the shape of the H/V spectra; in this region, the value of H/V spectral ratio remains nearly constant in the frequency of microtremors. We also estimated subsurface structures using the observed H/V spectra. Using a trial-and-error estimation process, S-wave velocity, P-wave velocity, and density were fixed, and the thickness of the sedimentary layers was adjusted to find a reasonable fit between the primary natural peak period of the calculated H/V spectra and the observed H/V spectra in order to determine the ground structure. The depth to the Vs = 600 m/s layer is estimated as 1.2 km at the boring site USDP2 that lies to the east of the Unzen volcanic area. Our result is consistent with borehole sample data..
7.	Kohno, Y., Matsushima, T., Shimizu, H., Magma Supply System of Unzen Volcano Inferred from Ground Deformation Data, 5th International UNESCO Conference on Geoparks, 3-P-12, 2012.05, Unzen had erupted in 1990~1995. The eruption had created a lava dome at the top of the mountain and caused several pyroclastic flows which killed 44 victims. Joint Research Team of National Universities and Geographical Survey Institution had observed ground deformation around Unzen for trying to predict the volcanic activity using leveling, GPS technique, tiltmeter and EDM (Electronic Distance Measurement). Especially, the leveling and GPS survey had been conducted from 1986 and 1996, respectively. During the lava discharging, a continuous deflation of the volcano was observed by them. After 1996, however, even lava had not been erupted from the crater, the leveling and GPS survey result had shown an inflation of western area of Shimabara peninsula. It could mean magma had kept to going up even after the eruption ceased. We estimate an appropriate source model for the deformation to estimate magma supply system of Unzen Volcano. Obtained sources are four, and they are arranged in ascending order toward the summit with the angle of 45~50 degree. Although this model has established by geodetic data, their positions are supported by seismic exploration researches. Moreover the upper boundary of these four sources corresponds to hypocenter of the earthquake swarms started in 1989, which were due to brittle-fracture around the magma path. From our result, after the eruption stopped magma inflows into deep magma chamber, located beneath Chijiwa bay at a depth of 15 km, is clarified. This means it is important to keep survey Unzen Volcano to understand its activity..
8.	Itoya, N., Matsushima, T., Estimation of subsurface structure using microtremor H/V spectral ratio in the Shimabara peninsula, 2010 AGU Fall Meeting, S51A-1908, 2010.12, The Shimabara peninsula in Kyushu island of Japan, located on the west edge of the Beppu Shimabara graben which crosses the center part in Kyushu from east to west. Seventy percent of the peninsula is covered with volcanic product of Unzen volcano. Using strong motion H/V spectral ratio, the Central Disaster Prevention Council (2008) pointed out that the long-period strong ground motions in the Shimabara peninsula are amplified so much like the Quaternary plains though the sedimentary layer of Quaternary Era is not thick in the peninsula. Especially, in the Unzen hot-spring region in the center part of the peninsula, the long-period strong ground motions amplify to the same extent as Kanto plains. It is thought that the thick volcanic deposit layer is a cause of the increase of the long-period ground motions. Then, the our research attempted the presumption of a peculiar amplification ground structure that was differed from the part of plains by paying attention to the microtremor as an evaluation method of the ground structure. The microtremor observations using three-components wideband seismometer were carried out at large number of sites in the Shimabara peninsula. The microtremor observation measured for about three days in each observation site by the seismometer of characteristic period 120 seconds. Power spectrum of UD, NS, and EW was calculated and smoothed using the ensemble average of thirty times, and power ratio of the horizontal and vertical spectrum (H/V spectrum ratio) was estimated. Here, the horizontal component was assumed to be a square root of the second power harmony of the NS component and the EW component. Peak frequency of 0.1-0.2Hz in the H/V spectrum ratio was obtained at a lot of observation sites at east side of the Shimabara peninsula. Volcanic sediments are thickly distributed in east side of the Shimabara peninsula. It is thought that the thick volcanic sediment layer is a cause of such lower peak frequency of H/V spectrum ratio. From our analysis using forward modeling S-wave velocity structure, the thickness of Vs=700m/s layer is estimated as 1.2km. The result is consists with nearby borehole data..

Classes taught:
Observational Volcanology
Mathematical Analysis Seminar for Seismology and Volcanology
Seminar of Seismological and Volcanological Instrumentation
Advanced Study of Earth and Planetary Sciences I, II
Advanced Seminar of Earth and Planetary Sciences I,II

Students supervised:
Two doctor-course students
Three master-course students
An undergraduate student

Extracurricular Activities:
Cooperation for a field trip of the undergraduate students

International Cooperation:
A supervisor of trainees in Volcanology Course of JICA