| 1. |
Haruo Sato, Kentaro Emoto, Propagation of a vector wavelet through von Kármán-type random elastic media: Monte Carlo simulation by using the spectrum division method, Geophysical Journal International, 10.1093/gji/ggad169, 234, 3, 1655-1680, 2023.04, SUMMARY
For the study of the random velocity fluctuation of the solid Earth medium, it is useful to measure the collapse of a seismic wavelet with increasing travel distance and the excitation of coda waves. Radiative transfer theory (RTT) is a powerful tool for synthesizing the propagation of a seismic wavelet in random media statistically characterized by the power spectral density function (PSDF) of the fractional velocity fluctuation. The Born scattering coefficient is a key building block of RTT. As the centre wavenumber of a wavelet increases, the phase shift across the correlation length increases and the Born approximation leads to an extremely large forward scattering exceeding the applicable range of the perturbation method. In such a case, the Eikonal approximation is able to explain the envelope broadening with increasing travel distance; however, it can not explain the coda excitation. To overcome the difficulty, we have proposed a hybrid Monte Carlo (MC) simulation for scalar waves. In the case of von Kármán-type random media, when the centre wavenumber is higher than the corner wavenumber, taking the centre wavenumber as a reference, we divide the PSDF into two spectral components. Applying the Born and Eikonal approximations to the high- and low-wavenumber spectral components, we statistically evaluate the wide-angle scattering and the narrow-angle ray bending, respectively. The proposed MC simulation serially using two kinds of scattering processes successfully synthesizes the time trace of the wave energy density from the onset to the late coda. The travel-distance fluctuation derived from the one-way propagation of the Eikonal approximation is also important. This paper extends this method for the propagation of a vector wavelet in random elastic media. We suppose that random fractional fluctuations of the P- and S-wave velocities and the mass density are linearly proportional to each other, which maintains the linear polarization of an Swave throughout the scattering process. Using the hybrid MC simulation with the spectrum division, we synthesize three-component energy density time traces for the anisotropic radiation from a moment tensor source, from which we derive three-component root mean square (RMS) velocity amplitude time traces at different azimuths. In parallel, we synthesize the propagation of a vector wavelet in many realized random elastic media by the finite-difference simulation, then we calculate three-component RMS velocity amplitude time traces. Using them as a benchmark, we confirm the validity of the proposed MC simulation for specific cases.. |
| 2. |
Shunsuke Takemura, Kentaro Emoto, Lina Yamaya, High-frequency S and S-coda waves at ocean-bottom seismometers, Earth, Planets and Space, 10.1186/s40623-023-01778-8, 75, 1, 20, 2023.02. |
| 3. |
Hisashi Nakahara, Kentaro Emoto, Takeshi Nishimura, Extending the formulation of the spatial autocorrelation (SPAC) method to strain, rotation and tilt, Geophysical Journal International, 10.1093/gji/ggab217, 227, 1, 287-302, 2021.06. |
| 4. |
Shunsuke Takemura, Satoru Baba, Suguru Yabe, Kentaro Emoto, Katsuhiko Shiomi, Takanori Matsuzawa, Source Characteristics and Along‐Strike Variations of Shallow Very Low Frequency Earthquake Swarms on the Nankai Trough Shallow Plate Boundary, Geophysical Research Letters, 10.1029/2022gl097979, 49, 11, 2022.06. |
| 5. |
Kentaro Emoto, Haruo Sato, Takeshi Nishimura, Synthesis of vector wave envelopes on the free surface of a random medium for the vertical incidence of a plane wavelet based on the Markov approximation, JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH, 10.1029/2009JB006955, 115, 8, B08306, 2010.08, In short-period seismograms of earthquakes, we often observe the broadening of apparent duration of P and/or S waves and the excitation of the transverse component especially for P waves as travel distance increases. Such phenomena are well explained by scattering caused by random velocity fluctuation in the lithosphere. The Markov approximation is known as one of the powerful stochastic methods for the direct synthesis of wave envelopes. We extend the method to synthesize vector wave envelopes on the free surface of a random medium since seismic observation is usually done on the ground surface. We evaluate the mean square (MS) envelope on the free surface by multiplying the amplification factor on the free surface to the angular spectrum in an infinite random medium. We synthesize MS envelopes for the vertical incidence of an impulsive plane P or S wavelet into a 3-D random medium characterized by a Gaussian autocorrelation function with typical parameters of the lithosphere. As a result, the vertical and horizontal component MS envelopes show different amplification rates on the free surface; however, we may say that "a factor of 4" is a good approximation for the amplification rate for both components. Finally, we numerically confirm the validity of our direct envelope syntheses by the comparison with finite difference simulations of waves in 2-D random media. The Markov approximation is accurate when the wavelength is shorter than the correlation distance and the fractional fluctuation is much smaller than the ratio of the correlation distance to the propagation distance.. |
| 6. |
Kentaro Emoto, Haruo Sato, Takeshi Nishimura, Synthesis of vector wave envelopes on the free surface of a random medium for the vertical incidence of a plane wavelet based on the Markov approximation, Journal of Geophysical Research: Solid Earth, 10.1029/2009JB006955, 115, 8, 2010.08, In short-period seismograms of earthquakes, we often observe the broadening of apparent duration of P and/or S waves and the excitation of the transverse component especially for P waves as travel distance increases. Such phenomena are well explained by scattering caused by random velocity fluctuation in the lithosphere. The Markov approximation is known as one of the powerful stochastic methods for the direct synthesis of wave envelopes. We extend the method to synthesize vector wave envelopes on the free surface of a random medium since seismic observation is usually done on the ground surface. We evaluate the mean square (MS) envelope on the free surface by multiplying the amplification factor on the free surface to the angular spectrum in an infinite random medium. We synthesize MS envelopes for the vertical incidence of an impulsive plane P or S wavelet into a 3-D random medium characterized by a Gaussian autocorrelation function with typical parameters of the lithosphere. As a result, the vertical and horizontal component MS envelopes show different amplification rates on the free surface
however, we may say that "a factor of 4" is a good approximation for the amplification rate for both components. Finally, we numerically confirm the validity of our direct envelope syntheses by the comparison with finite difference simulations of waves in 2-D random media. The Markov approximation is accurate when the wavelength is shorter than the correlation distance and the fractional fluctuation is much smaller than the ratio of the correlation distance to the propagation distance. Copyright 2010 by the American Geophysical Union.. |
| 7. |
Kentaro Emoto, Haruo Sato, Takeshi Nishimura, Synthesis and applicable condition of vector wave envelopes in layered random elastic media with anisotropic autocorrelation function based on the Markov approximation, GEOPHYSICAL JOURNAL INTERNATIONAL, 10.1111/j.1365-246X.2011.05264.x, 188, 1, 325-335, 2012.01, The Markov approximation is a powerful stochastic method for the direct synthesis of wave envelopes in random velocity fluctuated media when the wavelength is shorter than their correlation distance. To apply the Markov approximation to realistic cases, we consider horizontal layered random media characterized by an anisotropic autocorrelation function (ACF), where different layers have different randomnesses and different background velocities having step-like changes. Solving the parabolic master equation for the two frequency mutual coherence function (TFMCF) in random elastic media for the vertical incidence of a plane wavelet from the bottom, we calculate the angular spectrum just before the first velocity boundary. Multiplying transmission or PS conversion coefficients of the boundary by the angular spectrum, we calculate the angular spectrum and TFMCF on the other side of the boundary. Then we solve the master equation for the forward propagating wavelet in the second layer. Taking the same procedure for each layer boundary, we finally obtain the mean square (MS) vector envelopes on the free surface on the top layer. For the practical simulation, we use 2-D random media characterized by a Gaussian ACF. We numerically confirm the validity of the envelope synthesis for a specific case of layered random media with anisotropic ACF by comparing with finite difference (FD) simulations of elastic waves. Considering the Earth structure, the horizontal correlation distance is larger than the vertical one and the velocity fractional fluctuation becomes weak as depth increases, the Markov approximation is good for modelling the primary wavelet and also the converted wavelet for the vertical incident wavelet. We derive an applicable range of the Markov approximation for random media with anisotropic ACF by comparing with FD simulations. The results show that the Markov approximation is accurate when the wavelength is comparable or shorter than the both of vertical and horizontal correlation distances and the MS fractional fluctuation is much smaller than the ratio of squared horizontal correlation distance to the product of the vertical correlation distance and propagation distance.. |
| 8. |
Kentaro Emoto, Haruo Sato, Takeshi Nishimura, Envelope synthesis of a cylindrical outgoing wavelet in layered random elastic media based on the markov approximation, Geophysical Journal International, 10.1093/gji/ggt125, 194, 2, 899-910, 2013.07, In the heterogeneous earth medium, short period seismograms of an earthquake are wellcharacterized by their smooth envelopes with random phases. The Markov approximation hasoften been used for the practical synthesis of their envelopes for a given frequency band. Itis a stochastic extension of the phase screen method to synthesize wave envelopes in mediawith random fluctuations under the condition that the wavelength is shorter than the correlationdistance of the fluctuation. We propose an extension of the Markov approximation forthe envelope synthesis to the case that an isotropically outgoing wavelet is radiated from apoint source in horizontal layered random elastic media, where different layers have differentrandomness and different background velocities. In each layer, we solve the master equationfor the two frequency mutual coherence function (TFMCF) which contains the informationof the intensity in the frequency domain. Just below each layer boundary, we calculate theangular spectrumwhich is the expression of the TFMCF in the transverse wavenumber domainfor up-going wavelets. The angular spectrum shows the ray angle distribution of intensitiesof scattered waves. Multiplying it by the square of transmission coefficients calculated fromthe background velocity contrast at the boundary, we evaluate the angular spectrum just aboveit. We neglect P to S (S to P) conversion scattering inside of each layer
however, we takeinto account the mode conversion at the layer boundary. Different from the vertical incidenceof a plane wavelet, the wavefront expands with time and its curvature is modified at thelayer boundary due to the Snell's law. Approximating the wavefront in the second layer bya circle for a small incidence angle, we may shift the real origin to the pseudo-origin of thewavefront circle, which leads to the change in geometrical spreading factor. Finally, we calculatethe mean square envelope from the TFMCF by using an FFT. By multiplying the angularspectrum by conversion or reflection coefficients and calculate the TFMCF for the convertedor reflected wavelets at layer boundaries, we can calculate any phase generated due to velocitydiscontinuities. For the reflected wavelets, we solve the master equation of the TFMCF downward. To confirm the validity of the method, we directly synthesize mean square envelopesin 2-D two-layered random elastic media and compare them with the averaged envelopescalculated by finite difference (FD) simulations of the wave propagation in random elasticmedia. We find that the Markov envelopes well agree with the FD envelopes not only for atransmitted wavelet but also for a P to S converted wavelet and a reflected wavelet at a layerboundary.© The Authors 2013.. |
| 9. |
F. Brenguier, M. Campillo, T. Takeda, Y. Aoki, N. M. Shapiro, X. Briand, K. Emoto, H. Miyake, Mapping pressurized volcanic fluids from induced crustal seismic velocity drops, SCIENCE, 10.1126/science.1254073, 345, 6192, 80-82, 2014.07, Volcanic eruptions are caused by the release of pressure that has accumulated due to hot volcanic fluids at depth. Here, we show that the extent of the regions affected by pressurized fluids can be imaged through the measurement of their response to transient stress perturbations. We used records of seismic noise from the Japanese Hi-net seismic network to measure the crustal seismic velocity changes below volcanic regions caused by the 2011 moment magnitude (M-w) 9.0 Tohoku-Oki earthquake. We interpret coseismic crustal seismic velocity reductions as related to the mechanical weakening of the pressurized crust by the dynamic stress associated with the seismic waves. We suggest, therefore, that mapping seismic velocity susceptibility to dynamic stress perturbations can be used for the imaging and characterization of volcanic systems.. |
| 10. |
K. Emoto, M. Campillo, F. Brenguier, X. Briand, T. Takeda, Asymmetry of coda cross-correlation function: dependence of the epicentre location, GEOPHYSICAL JOURNAL INTERNATIONAL, 10.1093/gji/ggv081, 201, 3, 1313-1323, 2015.06, We analyze the cross-correlation function (CCF) of coda of earthquakes, which is used to retrieve the Green's function between two stations as well as the CCF of ambient noise. We select 74 Hi-net stations located in eastern Japan and 66 earthquakes to calculate the CCF. For each earthquake, we calculate the CCFs between possible pairs for the frequency bands of 0.1-0.2 Hz, 0.2-0.4 Hz and 0.4-0.8 Hz. Then we stack the CCFs for different earthquakes at each pair to obtain the average CCF. Although the correlation coefficients between the average and each CCFs are lower than 0.5 for most of the earthquakes, we obtain the propagating Rayleigh wave trace from average CCFs. We focus on the ratio of the amplitude in the positive lag time of the CCF to that in the negative lag time. CCFs for different earthquakes show different ratios which depend on the angle between the path of two stations and the epicentre. The amplitude in the lag time corresponding to the signal travelling from the near source station to the far source station is larger than that in the opposite lag time. Therefore the energy flux is not isotropic even in the coda and the energy from the source side is dominant. We average the ratios of pairs whose absolute values of angles are less than 45A degrees. The average ratios are 0.5 at 0.1-0.2 Hz. For higher frequencies, the ratio is not clear because of the bad signal-to-noise ratio. According to the diffusion model, the ratio is predicted as 0.6. Therefore, the coda is represented as the diffusion state in 0.1-0.2 Hz with our observation setting.. |
| 11. |
Haruo Sato, Kentaro Emoto, Synthesis of a scalar wavelet intensity propagating through von Kármán-type random media: joint use of the radiative transfer equation with the Born approximation and the Markov approximation, Geophys. J. Int., 10.1093/gji/ggx318, 211, 1, 512-527, 2017.10, 共著. |
| 12. |
Kentaro Emoto, Tatsuhiko Saito, Katsuhiko Shiomi, Statistical parameters of random heterogeneity estimated by analysing coda waves based on finite difference method, Geophys. J. Int., 211, 3, 1575-1584, 2017.09, 筆頭. |
| 13. |
Hisashi Nakahara, Kentaro Emoto, Deriving Sensitivity Kernels of Coda-Wave Travel Times to Velocity Changes Based on the Three-Dimensional Single Isotropic Scattering Model, PURE AND APPLIED GEOPHYSICS, 10.1007/s00024-016-1358-0, 174, 1, 327-337, 2017.01, Recently, coda-wave interferometry has been used to monitor temporal changes in subsurface structures. Seismic velocity changes have been detected by coda-wave interferometry in association with large earthquakes and volcanic eruptions. To constrain the spatial extent of the velocity changes, spatial homogeneity is often assumed. However, it is important to locate the region of the velocity changes correctly to understand physical mechanisms causing them. In this paper, we are concerned with the sensitivity kernels relating travel times of coda waves to velocity changes. In previous studies, sensitivity kernels have been formulated for two-dimensional single scattering and multiple scattering, three-dimensional multiple scattering, and diffusion. In this paper, we formulate and derive analytical expressions of the sensitivity kernels for three-dimensional single-scattering case. These sensitivity kernels show two peaks at both source and receiver locations, which is similar to the previous studies using different scattering models. The two peaks are more pronounced for later lapse time. We validate our formulation by comparing it with finite-difference simulations of acoustic wave propagation. Our formulation enables us to evaluate the sensitivity kernels analytically, which is particularly useful for the analysis of body waves from deeper earthquakes.. |
| 14. |
Haruo Sato, Kentaro Emoto, Synthesis of a scalar wavelet intensity propagating through von Kármán-type random media: Radiative transfer theory using the Born and phase-screen approximations, Geophys. J. Int., 10.1093/gji/ggy319, 215, 2, 909-923, 2018.08, 共著. |
| 15. |
Kentaro Emoto, Haruo Sato, Statistical characteristics of scattered waves in three-dimensional random media: comparison of the finite difference simulation and statistical methods, Geophys. J. Int., 10.1093/gji/ggy298, 215, 1, 585-599, 2018.07, 筆頭. |
| 16. |
Estimation of Statistical Parameters of Small-Scale Heterogeneities in the Crust by Analyzing the Spatial Variation of Seismic Wave Energy Field. |
| 17. |
3D Visualization of the Seismicity on the Mobile Device: Development of the “3D Quakes”. |
| 18. |
Keisuke Yoshida, Tatsuhiko Saito, Kentaro Emoto, Yumi Urata, Daisuke Sato, Rupture directivity, stress drop, and hypocenter migration of small earthquakes in the Yamagata-Fukushima border swarm triggered by upward pore-pressure migration after the 2011 Tohoku-Oki earthquake, Tectonophysics, 10.1016/j.tecto.2019.228184, 769, 228184, 2019.10, 共著. |
| 19. |
Shunsuke Takemura, Suguru Yabe, Kentaro Emoto, Modelling high-frequency seismograms at ocean bottom seismometers: effects of heterogeneous structures on source parameter estimation for small offshore earthquakes and shallow low-frequency tremors, 10.1093/gji/ggaa404, 223, 3, 1708-1723, 2020.09. |
| 20. |
Takeshi Nishimura, Kentaro Emoto, Hisashi Nakahara, Satoshi Miura, Mare Yamamoto, Shunsuke Sugimura, Ayumu Ishikawa, Tsunehisa Kimura, Source location of volcanic earthquakes and subsurface characterization using fiber-optic cable and distributed acoustic sensing system, Scientific Reports, 10.1038/s41598-021-85621-8, 11, 1, 2021.03. |
