Kiyoshi Yomogida |

Faculty of Science Earth and Planetary Sciences Earth and Planetary Dynamics |

Professor |

Last Updated :2020/09/25

- 1990 - 1998 Hiroshima University
- 1990 - 1998 Hiroshima University, Associate Professor
- 1987 - 1989 アメリカ合衆国スタンフォード大学 助教授
- 1987 - 1989 Stanford University, Assistant Professor
- 1986 - 1986 アメリカ合衆国スタンフォード大学 代理助教授
- 1986 - 1986 Stanford University,
- 1985 - 1986 アメリカ合衆国カリフォルニア工科大学 研究員
- 1985 - 1986 California Institute of Technology,
- Acting Assistant Professor
- Research Fellow

- - 1985 Massachusetts Institute of Technology
- - 1985 Massachusetts Institute of Technology Graduate School, Division of Earth Science Geophysics
- - 1980 The University of Tokyo Faculty of Science
- - 1980 The University of Tokyo Faculty of Science

- Ayumu Mizutani, Kiyoshi Yomogida, Yuichiro TaniokaJournal of Geophysical Research: Oceans 125 (9) 2169-9275 2020/09 [Refereed][Not invited]
**Quick estimation of wavefield by Neumann-type extrapolation for a new earthquake early warning system**Asuka Sato, Kiyoshi YomogidaProceeding of ECGS & ESC/EAEE Joint Workshop 31 59 - 73 2016 [Refereed][Not invited]- Masashi Ogiso, Kiyoshi YomogidaJOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 298 15 - 26 0377-0273 2015/06 [Refereed][Not invited]

In the early morning on 16 October 2013, large debris flows resulted in over 30 people dead on Izu-Oshima Island, Japan, which were induced by heavy rainfall from the approaching Typhoon 1326 (Wipha). We successfully estimated the locations and migration processes of five large events of the debris flows, using the spatial distribution of high-frequency seismic amplitudes recorded by a seismic network on the island. The flows occurred on the western flank of the island, almost at the same place as the site where large traces of debris flows were identified after the disaster. During each event of debris flows, the estimated locations migrated downstream with increasing time, from the caldera rim of Miharayama volcano in the center of the island to its western side with a speed of up to 30 m/s. The estimated time series of source amplitudes are different from event to event, exhibiting a large variety of flow sequences while they seem to have repeated at a relatively narrow area over several tens of minutes. The present approach may be utilized for early detection and warning for prevention and reduction of the present type of disasters in the future. (C) 2015 Elsevier B.V. All rights reserved. **Variability of megathrust earthquakes in the world revealed by the Tohoku-oki megathrust earthquake on 11 March, 2011**Koyama J, Tsuzuki M, Yomogida K, Yoshizawa KGeophy Bull Hokkaido Univ 76 129 - 146 2013/03 [Not refereed][Not invited]

(in Japanese with English abstract)- Masashi Ogiso, Kiyoshi YomogidaJOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 217 8 - 20 0377-0273 2012/03 [Refereed][Not invited]

We estimate the locations of three tremor sequences, denoted A, B, and C, that occurred before the 2008 eruption at Meakandake volcano, eastern Hokkaido, Japan, using the spatial distribution of seismic amplitudes of volcanic tremor. Although we used only five seismic stations, we could estimate the locations of three tremor sequences. We find two different tremor source areas: those of Tremor A located about 2 km southwest of the erupted crater (Area A) and the other about 1 km southeast to south of the crater as that of the Tremor C (Area B). The location of an early phase of the Tremor B is estimated in Area A while the location of its later phase appears to connect the two areas. This location migration during Tremor B occurred simultaneously with other important geophysical phenomena before the eruption event, as suggested by geodetic and geomagnetic studies. Our findings of the two tremor locations of the tremor sequences and location migration during Tremor B may be important for future monitoring activities at Meakandake volcano, particularly to forecast the location of a possible phreatic eruption. (C) 2011 Elsevier B.V. All rights reserved. - Junji Koyama, Kazunori Yoshizawa, Kiyoshi Yomogida, Motohiro TsuzukiEARTH PLANETS AND SPACE 64 (12) 1189 - 1198 1343-8832 2012 [Refereed][Not invited]

The seismicity of the Pacific coast of Tohoku, Japan, has been investigated in detail and characterized into regional seismic segments. The 2011 megathrust earthquake of M-w 9.0 on 11 March ruptured almost all of the segments in that area, causing devastating tsunamis. The prime factor that had not been recognized before is the double segmentation along the Japan trench: The apparent absence of earthquakes in the trench-ward segments as opposed to the Japan Island-ward segments that have repeated smaller earthquakes. We term this pattern of seismic activity 'along-dip double segmentation (ADDS)'. The 2011 Tohoku megathrust earthquake is typical of a class of great earthquakes different from that of the 1960 Chile earthquake, in which a young and buoyant plate is subducting rapidly under the continental plate. In the 1960 Chile case, the seismic activity is characterized by 'along-strike single segmentation (ASSS)', where there is weak seismic activity before the main event all over the plate interface of the subduction zone. We study the greatest earthquakes around the world and find that there is a variety of megathrust earthquakes characterized by ASSS to ADDS, where the 2004 Sumatra-Andaman, the 1960 Chile, the 1964 Alaska and the 2011 Tohoku-oki earthquakes are typical end-members. - Kiyoshi Yomogida, Kazunori Yoshizawa, Junji Koyama, Motohiro TsuzukiEARTH PLANETS AND SPACE 63 (7) 697 - 701 1343-8832 2011 [Refereed][Not invited]

We present some singular characteristics of the 2011 off the Pacific coast of Tohoku Earthquake in comparison with other megathrust earthquakes, such as the 1960 Chilean and the 2004 Sumatra-Andaman earthquakes. In addition to the conventional along-strike segmentation, along-dip segmentation of the fault area or subduction zone is an important feature for the Tohoku subduction zone, as indicated by the difference in background seismicity: virtually no seismicity in shallow segments but active with large events repeating in deep segments. The interaction between along-dip segments (deep and shallow segments) led to the great 2011 Tohoku earthquake. The along-dip segmentation results in plane or two-dimensional rupture propagation on a coseismic fault. Significant along-strike variability is also important for the 2011 Tohoku earthquake, with segments of both weak (e. g., slow or tsunami earthquakes) and strong plate couplings located adjacent to each other. In contrast, every segment appears to be with strong plate coupling for other megathrust earthquakes. One exception is the 1964 Alaska earthquake that shares a certain degree of common characteristics with the 2011 Tohoku earthquake: two distinct seismogenic zones along the dip direction of the trench, that is, the along-dip segmentation is noticeable. Significant along-strike variability also characterizes the activities in and around the subduction zone of the 1964 Alaska earthquake, including a creeping segment and a tsunami-earthquake segment. - BOUROVA E, YOSHIZAWA K, YOMOGIDA KPhys Earth Planet Interiors 183 (1-2) 20-32 - 32 0031-9201 2010/11 [Refereed][Not invited]
- Yoshizawa, K, Miyake, K, Yomogida, KPhys. Earth Planet. Inter. 183 (1-2) 4 - 19 0031-9201 2010 [Refereed][Not invited]
- Osada, K, Yoshizawa, K, Yomogida, KPhys. Earth Planet. Inter. 183 (1-2) 63 - 72 0031-9201 2010 [Refereed][Not invited]
- R. Yamada, I. Yamada, H. Shiraishi, S. Tanaka, Y. Takagi, N. Kobayashi, N. Takeuchi, Y. Ishihara, H. Murakami, K. Yomogida, J. Koyama, A. Fujimura, H. MizutaniPlanetary and Space Science 57 (7) 751 - 763 0032-0633 2009/06 [Refereed][Not invited]

We developed a seismometer system for a hard landing "penetrator" probe in the course of the former Japanese LUNAR-A project to deploy new seismic stations on the Moon. The penetrator seismometer system (PSS) consists of two short-period sensor components, a two-axis gimbal mechanism for orientation, and measurement electronics. To carry out seismic observations on the Moon using the penetrator, the seismometer system has to function properly in a lunar environment after a hard landing (impact acceleration of about 8000 G), and requires a signal-to-noise ratio to detect lunar seismic events. We evaluated whether the PSS could satisfactorily observe seismic events on the Moon by investigating the frequency response, noise level, and response to ground motion of our instrument in a simulated lunar environment after a simulated impact test. Our results indicate that the newly developed seismometer system can function properly after impact and is sensitive enough to detect seismic events on the Moon. Using this PSS, new seismic data from the Moon can be obtained during future lunar missions. © 2008 Elsevier Ltd. All rights reserved. - Jun Kawahara, Taichi Ohno, Kiyoshi YomogidaJournal of the Acoustical Society of America 125 (6) 3589 - 3596 0001-4966 2009 [Refereed][Not invited]

Propagation and scattering of antiplane shear waves within media with two-dimensional cavities are numerically simulated, and the attenuation and phase velocities are experimentally determined. The results are compared with the predictions by the Foldy theory and its three corrected versions. If the cavity concentrations are small such as 0.02, the differences among the theoretical predictions are insignificant, and every theory is consistent with the experimental results. For higher concentrations such as 0.1, the differences become significant, but there are no objective grounds to say that any of the corrected versions of the Foldy theory works better than the original. If the error tolerance is as high as 10%, the simple Foldy formula may remain useful for concentrations up to about 0.1. © 2009 Acoustical Society of America. - T. Taira, K. YomogidaJournal of Geophysical Research: Solid Earth 112 (6) 2169-9356 2007/06/04 [Refereed][Not invited]

This is the first paper in a two-part series on a newly developed imaging approach for small-scale heterogeneities (< 1 km) in the crust with effects of scattering modes. To obtain a reliable crustal heterogeneous structure, we follow the six major steps: (1) removing overall complex propagation effects, including anelastic attenuation, by using a statistical technique with the use of the Akaike Information Criterion (AIC), (2) obtaining high-resolution frequency-wave number (f-k) power spectra and slowness vectors of spectral peaks in the time-frequency domain, based on a stationary autoregressive model, (3) estimating polarization vectors of the scattered waves identified in step 2 with a stationary multivariate autoregressive model, (4) determining scattering modes (i.e., P or S wave arrival) from the angle between the slowness and polarization vectors obtained in steps 2 and 3, respectively, (5) correcting effects of seismic-source radiation and surface geology by a coda-normalization approach, and finally (6) mapping the f-k power spectra into small blocks in a model space as scattering coefficients, using a slowness-weighted back-projection. We can incorporate scattering modes as well as propagation effects such as anelastic attenuation factors in the background medium, with the AIC based amplitude recovery technique. The resolution in f-k spectrograms and the accuracy of polarization estimates are significantly improved through the present approach, so that not only more scattered phases are clearly identified but also their spatial three-dimensional locations are pinpointed more precisely and stably than previous approaches in imaging based on scattering theory. Copyright 2007 by the American Geophysical Union. **Seismic exploration at Usu volcano with active sources - The outline of the experiment and the first arrival time data -**ONIZAWA Shin'ya, OSHIMA Hiromitsu, AOYAMA Hiroshi, MORI Hitoshi Y, MAEKAWA Tokumitsu, SUZUKI Atsuo, OKADA Hiromu, TSUTSUI Tomoki, MATSUWO Norimichi, OIKAWA Jun, OHMINATO Takao, YAMAMOTO Keigo, MORI Takehiko, TAIRA Takaaki, MIYAMACHI Hiroki, KOYAMA Junji, YOMOGIDA Kiyoshi, WATANABE Kenji, MATSUBARA Wakana, OKADA Jun, MIYAMURA Jun'ichi, TANIGUCHI Masami, YOSHIKAWA Akifumi, KATOH Kouji, HAMAGUCHI Hiroyuki, TANAKA Satoru, YAMAZAKI Jun, FUJISAWA Hiroatsu, OGAWA Yasuo, NOGAMI Kenji, SAITOH Akira, MIZUHASHI Shoei, WATANABE Hidefumi, KAGIYAMA Tsuneomi, HAGIWARA Michinori, NAKAMICHI Haruhisa, OSADA Noboru, KOYAMA Etsuro, IMOTO Yoshiko, KOBAYASHI Tomokatsu, HIYAMA Youhei, SUGIOKA Manabu, ISHIMINE Yasuhiro, YAMAOKA Kousyun, OKUDA Takashi, KOIKE Katsuhiko, SUZUKI Takayuki, TSURUGA Kayoko, SHIMIZU Hiroshi, MATSUMOTO Satoshi, MATSUMOTO Kaoru, OHKURA Takahiro, SAKO Mikio, YOSHIKAWA Shin, YAKIWARA Hiroshi, HIRANO Shuichiro, HAYASHIMOTO Naoki, TAMEGURI TakeshiBulletin of the Earthquake Research Institute, University of Tokyo 78 (2) 121 - 143 0040-8972 2003/09 [Refereed][Not invited]- T Taira, K YomogidaBULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 93 (4) 1531 - 1541 0037-1106 2003/08 [Refereed][Not invited]

We estimated the spatial distribution of small-scale heterogeneities as anomalous amplification of coda level in the Hidaka, Japan, region. We analyzed 2768 seismograms in a frequency range of 1-32 Hz for 24 earthquakes recorded at 62 stations. First, we estimated the site effect of each station, using the coda normalization method with regional earthquake data of large epicentral distances. All the data processing was done after the correction of this site effect. Next, we determined coda amplitude factor (CAF), that is, the amplitude ratio of coda waves on each source-station pair relative to the averaged coda amplitude over stations, for earthquakes inside the region. We found systematic spatial variations of CAF, implying the existence of localized heterogeneities. At frequencies lower than 2 Hz, the CAF is relatively large in the west of the Hidaka Mountains, implying the possibility of strong heterogeneities with a scale of 0.6-1.3 km there. In a high-frequency range (>16 Hz), CAF values are large on paths crossing the Hidaka Mountains. From the corresponding lapse time of coda (65 sec), there may be a zone of concentrated heterogeneities beneath the Hidaka Mountains at the depth of 100-120 km. - Tono, Y., Yomogida, K.Physics of the Earth and Planetary Interiors 103 (1-2) 1 - 16 0031-9201 1997 [Refereed][Not invited]

- 共立出版 2007 (ISBN: 9784320018297)
**演習形式で学ぶ特殊関数・積分変換入門** - Kyoritsu Shuppan 2007 (ISBN: 9784320018297)
**Self-study guide to special functions and integral transforms** - Lecture notes of JICA training course in Seismology and earthquake engineering 1993
**Surface waves, and Scattering and attennation of seismic waves**

- 2000
**2000年有珠山噴火の広帯域地震計観測** - 2000
**Broadband seismic observation of the 2000 rolcanic eruption of Mt. Usn.** - 1995
**1995年兵庫県南部地震の広帯域地震計観測** - 1995
**1995年兵庫県南部地震の地震断層** - 1995
**Broadband seismic observation of the 1995 Hyogoken-nanbu earthquake** - 1995
**Surface fault ruptures associated with the 1995 Hyogoken-nanbu earthquake** - 1993
**1990年ルソン地震に伴なう地表面破壊の大きなすべり速度** - 1993
**Large slip velocity of the surface rupture associated with the 1990 Luzon earthquake**

- Hiroo Kanamori, Kiyoshi Yomogida EARTH PLANETS AND SPACE 63- (7) 511 -511 2011 [Not refereed][Not invited]
- T. Taira, K. Yomogida, Y. Kuwahara, K. Imanishi, H. Ito JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 112- (B6) B06312 2007/06 [Not refereed][Not invited]

[1] This is the second paper in a two-part series on a newly developed imaging approach for small-scale heterogeneities (< 1 km) in the crust with effects of scattering modes. In the present paper, we estimated a detail three-dimensional spatial distribution of small-scale heterogeneities around the Nagamachi-Rifu fault, northeastern Japan, in a frequency range of 2 - 16 Hz, using the imaging approach presented in the first paper. We used seismograms recorded by dense three-component seismic arrays that were deployed in this region for 15 explosion sources. As one of our important results, there are concentrations for only P-S but not P-P scatterers near the surface trace of the fault. P-S scatterers in a frequency range of 8 - 16 Hz are localized near the surface trace of the fault, implying the possibility of strong heterogeneities with a size of 0.08 km there. In the Shirasawa caldera region, the characteristics of seismic scatterers seem to convert from large P-S to large P-P relative scattering coefficients with its transition depth range of 5 - 8 km. This feature implies that the materials composed of seismic scatterers may show a systematic variation with depth. Finally, the strength of scattering coefficients is rather weak in the coseismic area of the 15 September 1998 earthquake with a magnitude of 5.2, the largest recent event in this area. This result suggests that this coseismic area is rather homogeneous and can hold local stress larger than in the surrounding portions of the fault system. - T. Taira, K. Yomogida JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH 112- (B6) B06311 2007/06 [Not refereed][Not invited]

[1] This is the first paper in a two-part series on a newly developed imaging approach for small-scale heterogeneities (< 1 km) in the crust with effects of scattering modes. To obtain a reliable crustal heterogeneous structure, we follow the six major steps: ( 1) removing overall complex propagation effects, including anelastic attenuation, by using a statistical technique with the use of the Akaike Information Criterion (AIC), ( 2) obtaining high-resolution frequency-wave number (f-k) power spectra and slowness vectors of spectral peaks in the time-frequency domain, based on a stationary autoregressive model, ( 3) estimating polarization vectors of the scattered waves identified in step 2 with a stationary multivariate autoregressive model, ( 4) determining scattering modes (i.e., P or S wave arrival) from the angle between the slowness and polarization vectors obtained in steps 2 and 3, respectively, ( 5) correcting effects of seismic-source radiation and surface geology by a coda-normalization approach, and finally ( 6) mapping the f-k power spectra into small blocks in a model space as scattering coefficients, using a slowness-weighted back-projection. We can incorporate scattering modes as well as propagation effects such as anelastic attenuation factors in the background medium, with the AIC based amplitude recovery technique. The resolution in f-k spectrograms and the accuracy of polarization estimates are significantly improved through the present approach, so that not only more scattered phases are clearly identified but also their spatial three-dimensional locations are pinpointed more precisely and stably than previous approaches in imaging based on scattering theory. - M Ogiso, K Yomogida, K Katsumata EARTH PLANETS AND SPACE 57- (6) 477 -489 2005 [Not refereed][Not invited]

A new recursive inverse scheme is applied to a currently popular problem named seismic travel-time tomography, in order to enhance the efficiency and reliability in obtaining a new velocity model if a small number of new data are added to a large data set in the past. In comparison with conventional inverse schemes in seismic tomography, either least-squares or iterative types, this scheme does not require large amounts of matrix-type computations but utilizes the amount of modification in model parameters responsible for each new data set. We also introduce the computation of a collocation travel time (i.e., from a given station to every grid point) for the reference velocity model inverted by the data for all the past events, using a ray tracing scheme called the Huygens' method (Saito, 2001), suitable to computations prior to a new event. Combining the above information already stored with the recursive inverse scheme, we can obtain a new or updated velocity model immediately after a new event takes place, because a temporal interval between two events is usually very long in a given local area. Since the model is revised at each recursive step, we perform ray tracings with the updated reference model to get more accurate ray paths and travel times than the conventional inversion schemes that use all the ray tracings for the same reference model. We first showed the validity and stability of the proposed method with synthetic data. We then applied the new approach to the P-wave travel-time data recorded in the Hidaka, south-central Hokkaido, Japan, region, and compared our result with other previous results. Our result shares the overall feature with the previous ones. In addition, a new low-velocity zone is detected in the east of the Hidaka mountains at the depth of 10 km, corresponding to the collision zone of two arcs, due to the use of the updated reference velocity model at each recursive step. We also confirmed that the order of data does not affect the final result, so that the present approach is shown as an appropriate tool for so-called real-time seismic tomography: a updated velocity model is immediately obtained at each time that a new event takes place, in order to monitor temporal variations of model parameters such as velocity structure on the real-time basis. - T Taira, K Yomogida GEOPHYSICAL JOURNAL INTERNATIONAL 158- (3) 998 -1008 2004/09 [Not refereed][Not invited]

While large-scale heterogeneities have been studied intensively by seismic tomography with traveltime data, small-scale heterogeneities are not fully investigated yet. We obtained the 3-D spatial distribution of small-scale heterogeneities as relative scattering coefficients in the Hidaka, Japan, region using the spatial variation of high-frequency coda power spectra. We analysed 530 seismograms recorded at 62 stations for nine local earthquakes in the frequency range of 1-32 Hz. After correcting source, station and propagation effects, we assigned the spatial and temporal variations of observed power spectra of coda waves into the 3-D distribution of scattering coefficient, based on single and isotropic scattering models. Small-scale heterogeneities are distributed not only in a non-uniform manner but also differently among scale lengths estimated from their frequency dependence. An area of large relative scattering coefficient in 2 and 16 Hz is located at a depth shallower than 45 km in the southwest of the Hidaka Mountains, correlating well with a highly active area of microearthquakes. At a depth of 60-120 km, two dipping zones of large scattering coefficient are imaged beneath the summit of the Hidaka Mountains. These zones may imply two layers of strong seismic-wave reflection, consistent with the upper and lower planes of the double seismic zone in the subducting Pacific Plate, as determined by the hypocentral distribution in this region. - W Matsubara, K Yomogida, J Koyama, M Kasahara, M Ichiyanagi, H Kawakatsu, M Yamamoto JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 136- (1-2) 141 -158 2004/08 [Not refereed][Not invited]

We installed five broadband seismometers around Mt. Usu, Hokkaido, Japan, just before the first surface eruption on March 31, 2000. By using these broadband data with short-period seismograms recorded by Japan Meteorological Agency and Institute of Seismology and Volcanology of Hokkaido University, we located 590 earthquakes associated with the 2000 eruption of Mt. Usu from March 31, 12:58 to April 5, 2000, including 41 low-frequency earthquakes. Low-frequency earthquakes have clear predominant frequencies of about 1.0-1.5 Hz. While the seismicity of tectonic earthquakes was still active on April 5, there were almost no low-frequency events after April 2. All the low-frequency earthquakes occurred within a vertical and narrow zone near the craters of eruption, and they are shallower than 4 km. In contrast, the active area of tectonic earthquakes spreads out in a large area, particularly deep in the south of the craters. From these temporal and spatial patterns, together with GPS and gravity measurements that support the termination of magma activities before April 2, we suggest that low-frequency earthquakes were caused by magma or hydrothermal activities beneath the volcano while tectonic ones were caused by the regional tectonic stress affected by the intrusion of magma body near the craters. Related to long-term magma activities of this eruption, four deep crustal low-frequency events were identified on October 17, 1998 just in the south of Mt. Usu in the depth range of 20-30 km. As proposed for other active volcanoes, these deep crustal events may represent a part of deep magma activities beneath Mt. Usu. (C) 2004 Elsevier B.V All rights reserved. - W Matsubara, K Yomogida JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 134- (3) 223 -240 2004/06 [Not refereed][Not invited]

We installed five broadband seismometers around Mt. Usu just before the first eruption in 2000 and the observation continued over 8 months. Among the total of 590 relocated earthquakes that occurred from March 31 12:58 p.m. to April 5, 2000, 41 low-frequency earthquakes were identified. Low-frequency earthquakes, based on waveforms and S-wave spectra, can be classified into several families. We investigated the source process of the low-frequency earthquakes classified as "type 1" for which the similarity among events was the highest. The character of waveforms and S-wave spectra of type 1 earthquakes at Mt. Usu are quite similar to the "hybrid" and "LP" earthquakes at Mt. Redoubt, Alaska, whose source process was explained well by the resonance of a fluid-filled crack. Assuming that they have similar source processes, we estimated several parameters such as crack size by forward modeling. We obtained the following parameters that explain main features (e.g., the frequencies of major peaks) of the observed S-wave spectra at three stations (HGSN, SHNZN and ZNKJI) in the frequency range of 0.5-5 Hz, assuming a vertical crack. Crack length is 200 m, 100 m for its width, 0.14 m for its thickness, 100 for its stiffness, 0.5 for the ratio of bulk modulus and rigidity, and 0.07 for viscous damping loss. The pressure pulse triggering the crack oscillation has a rise time of 0.25 s. From the difference in characteristics of waveforms among stations of various azimuths, the strike of the crack was likely to be northeast-to-southwest. The use of the site-corrected spectra enables us to pin-point the value of viscous damping loss, so that we can conclude that water-steam mixture was responsible for the low-frequency events at Mt. Usu. (C) 2004 Elsevier B.V. All rights reserved. - R Honda, K Yomogida GEOPHYSICAL JOURNAL INTERNATIONAL 154- (2) 441 -462 2003/08 [Not refereed][Not invited]

While detailed crustal structure around a damaged area is surveyed for source parameters obtained by seismic waveform inversions (e.g. the 'damage belt' of the 1995 Hyogoken-nambu earthquake in Japan), the effects of a layered medium are often not considered in practical analyses of geodetic data. In this study, we calculate static as well as dynamic displacements with the discrete wavenumber method and compare them with analytical as well as numerical solutions for a half-space. The existence of a soft surface layer is found to affect both the amplitude and the spatial distribution of static displacement near a fault, in a different manner from those of dynamic displacement. The largest effect of a surface layer is shown in the case of a vertical strike-slip fault. While dynamic motions are amplified monotonically with the thickness of the surface layer, the amplification of static displacement is limited if the surface layer reaches the top of the fault because the moment release there is then reduced. Errors in the estimation of the fault depth and the amount of slip reaches as large as 30 per cent of the true values in the worst case. Our results strongly suggest that a strong bias may be introduced to the conventional estimation of fault parameters, assuming a medium to be a half-space, from static displacement or geodetic data by the existence of a low-velocity or low-rigidity surface layer. - Yoshizawa K, Yomogida K, Tsuboi S IUGG report "Geosciences: The Future" 11 -21 2003/07 [Refereed][Invited]
**Seismology and Physics of the Earth's Interior** - R Honda, K Yomogida PHYSICS OF THE EARTH AND PLANETARY INTERIORS 137- (1-4) 107 -127 2003/05 [Not refereed][Not invited]

When we represent seismic waves by standard wavenumber decomposition schemes such as the discrete wavenumber method, there is ambiguity in the choice of one parameter, the truncate number of the wavenumber integration. Although there is a conventional truncation number to obtain accurate seismograms, few studies have paid attention to seismograms including static displacement (i.e. w = 0). We estimate this parameter in order to accurately calculate not only body waves like P- and S-wave but also static displacement, by overcoming the poor convergence of integration over horizontal wavenumbers. In order to find a suitable value of the truncation number of wavenumbers, we utilize wavenumber-domain solutions from spatial-domain solutions for a homogeneous half space, using the analytic formulation of Okada [Bull. Seismol. Soc. Am. 75 (1985) 1135-1154] and estimate a region where spectral energy is concentrated. We shall identify the range in which the energy is reduced into 1/100 of the maximum amplitude in k(x) and k(y), and define the larger of k(x) and k(y) in this estimation as the truncation number of wavenumbers, k(max). For a finite rectangular fault buried in a half space, we find that a suitable value of the truncation number of wavenumber k(max) is 4 km(-1) by comparing the analytical solution of Okada for static deformation. If there is large moment release near the surface (e.g. artificial source such as explosion of dynamites), the convergence over horizontal wavenumbers becomes very poor not only for static but also for dynamic motions. Our results show that 4 km(-1) is a reasonable value when a fault extend to several kilometers under the surface even if the moment release at each, point on the fault surface is constant. In a realistic case, earthquake source has large moment release at several kilometers under the surface, so we can obtain surface displacement accurately, including static displacement in any realistic cases, with the above value of k(max). (C) 2003 Elsevier Science B.V All rights reserved. - R Honda, K Yomogida GEOPHYSICAL JOURNAL INTERNATIONAL 152- (2) 443 -454 2003/02 [Not refereed][Not invited]

Representing seismic waves numerically by, for example, the discrete wavenumber method, we normally decompose waves into P-SV and SH components. This scheme, however, cannot treat the exactly vertically travelling S-wave element (VTSE), which corresponds to k(x) = k(y) = 0, whereas VTSE exists except for a few cases of fault geometry. In order to deal with VTSE from a point source, a new S-wave potential must be introduced, so that its polarization is restricted on a horizontal plane. The polarization angle of VTSE is a function of the fault geometry represented by dip, rake and strike. In this study, we first point out that VTSE from a finite fault cannot be calculated even if we use the above new S-wave potential, because the terms originated from fault finiteness become zero if k(x) = k(y) = 0. In order to remedy this difficulty for a finite source, we define a new VTSE potential for a rectangular fault. The contribution of VTSE is large for displacements in a very low frequency range such as static displacement. The static displacement due to VTSE does not depend on a fault depth or station locations. When we take a small but finite value for k(x) and k(y) instead of zero, say deltak = 10(-5), we can obtain practically accurate results even without the VTSE potential, including static displacements that have not been discussed in detail yet. Following the above scheme, accurate seismograms with both dynamic and static components can be simulated simultaneously for a fault of any configuration and station location. - Effect of complex fault geometry and slip style on near-fault strong motions and static displacementR Honda, K Yomogida EARTH PLANETS AND SPACE 55- (9) 515 -530 2003 [Not refereed][Not invited]

Although there are many studies that deal with complex slip distribution or rupture propagation on an earthquake fault, they usually regard a fault system as a fault of simple geometry. Actual fault systems have highly heterogeneous slip distribution and very complicated shapes, as is often observed through field surveys of surface breaks. In this study, we synthesize seismograms including static displacement near a fault using the discrete wavenumber method in order to estimate the effects of the above types of fault complexity in a quantitative manner. We introduce a complex slip distribution based on the Nojima Fault associated with the 1995 Hyogo-ken Nanbu earthquake. As a result, we show that strong motions at a frequency of lower than 1.0 Hz are strongly affected by the complexity of the fault geometry, at a scale of not more than several km, rather than the rupture propagation style. Distributions of static displacement fluctuate, depending on the fault geometry characterized by the length of each fault segment. Such small-scale variations in fault geometry (less than or equal to1 km) have been mostly ignored prior to this work. Our results also suggest that details of fault segmentation and bending can be determined by dense observations (e.g., GPS or geological surveys) of static displacement near a fault system, indicating the importance of simultaneous studies on static and dynamic near-fault motions. - Taka'aki Taira, Kiyoshi Yomogida Bulletin of the Seismological Society of America 93- (4) 1531 -1541 2003 [Not refereed][Not invited]

We estimated the spatial distribution of small-scale heterogeneities as anomalous amplification of coda level in the Hidaka, Japan, region. We analyzed 2768 seismograms in a frequency range of 1-32 Hz for 24 earthquakes recorded at 62 stations. First, we estimated the site effect of each station, using the coda normalization method with regional earthquake data of large epicentral distances. All the data processing was done after the correction of this site effect. Next, we determined coda amplitude factor (CAF), that is, the amplitude ratio of coda waves on each source-station pair relative to the averaged coda amplitude over stations, for earthquakes inside the region. We found systematic spatial variations of CAF, implying the existence of localized heterogeneities. At frequencies lower than 2 Hz, the CAF is relatively large in the west of the Hidaka Mountains, implying the possibility of strong heterogeneities with a scale of 0.6-1.3 km there. In a high-frequency range (> 16 Hz), CAF values are large on paths crossing the Hidaka Mountains. From the corresponding lapse time of coda (65 sec), there may be a zone of concentrated heterogeneities beneath the Hidaka Mountains at the depth of 100-120 km. - Bulletin of Seismological Society of America 93, 1516-1530- 2003 [Not refereed][Not invited]
**Coda level amplification of aftershocks of the 1995 Hyogoken Nanbu earthquake** - K Yomogida, R Okuyama, Nakanishi, I STUDIA GEOPHYSICA ET GEODAETICA 46- (4) 691 -710 2002/10 [Not refereed][Not invited]

A clear later phase of amplitude larger than the direct surface wave packet was observed at stations in Hokkaido, Japan, for several events of the December 1991 off-Urup earthquake swarm in the Kuril Islands region. From its particle motion, this phase is likely to be a fundamental Rayleigh wave packet that arrived with an azimuth largely deviated from each great-circle direction. As its origin, Nakanishi ( 1992) proposed that the sea-trench topography in this area as deep as 10 km may produce a narrow zone of low velocity for Rayleigh waves of periods around 15 sec. Following this idea, we compute ray paths and estimate how Rayleigh waves would propagate if we assume that lateral velocity variations are caused only by seafloor topography. We confirm that thick sea water in the trench indeed produces the phase velocity of Rayleigh waves to be smaller than in a surrounding area by the degree over 100%. Such a low-velocity zone appears only in a period range from 12 to 20 sec. Although this strong low-velocity zone disturbs the direction of Rayleigh wave propagation from its great circle, the overall ray paths are not so affected as far as an epicentre is outside this low-velocity zone, that is, off the trench axis. In contrast, the majority of rays are severely distorted for an event within the low-velocity zone or, in other words, in the neighborhood of the trench axis. For such an event, a part of wave energy appears to be trapped in this zone and eventually propagates outwards due to the curvature or bend of trench geometry, resulting in very late arriving waves of large amplitude with an incident direction clearly different from great circles. This phenomenon is observed only at a very limited period range around 16 sec. These theoretical results are consistent with the above mentioned observation of Nakanishi (1992). - K Yomogida, R Benites PURE AND APPLIED GEOPHYSICS 159- (7-8) 1771 -1789 2002/07 [Not refereed][Not invited]

We develop a new scheme to compute 2-D SH seismograms for media with many flat cracks, based on the boundary integral method. A dry or traction-free boundary condition is applied to crack surfaces although other kinds of cracks such as wet or fluid-saturated cracks can be treated simply by assigning different boundary conditions. While body forces are distributed for cavities or inclusions to express scattered wave, dislocations (or displacement discontinuities between the top and the bottom surfaces of each crack) are used as fictitious sources along crack surfaces. With these dislocations as unknown coefficients, the scattered wave is expressed by the normal derivative of Green's function along the crack surface, which is called "double-layer potentials" in the boundary integral method, while we used "single-layer potentials" for cavities or inclusions. These unknowns are determined so that boundary conditions or crack surfaces are satisfied in the least-squared sense, for example, traction-free for dry cracks. Seismograms with plane-wave incidence are synthesized for homogeneous media with many cracks. First, we check the accuracy of our scheme for a medium with one long crack. All the predicted phases such as reflected wave, diffraction from a crack tip and shadow behind the crack are simulated quite accurately, under the same criterion as in the case for cavities or inclusions. Next, we compute seismograms for 50 randomly distributed cracks and compare them with those for circular cavities. When cracks are randomly oriented, waveforms and the strength of scattering attenuation are similar to the cavity case in a frequency range higher than kd similar or equal to 2 where the size of scatterers d (i.e., crack length or cavity diameter) is comparable with the wavelength considered (k is the wavenumber). On the other hand, the scattering attenuation for cracks becomes much smaller in a lower frequency range (kd < 2) because only the volume but not detail geometry of scatterers becomes important with wavelength much longer than each scatterer. When all the cracks are oriented in a fixed direction, the scattering attenuation depends strongly on the incident angle to the crack surface as frequency increases (kd > 2): scattering becomes weak for cracks oriented parallel to the direction of the incident wave, while it gets close to the cavity case for cracks aligned perpendicular to the incident wave. - M Yamamoto, H Kawakatsu, K Yomogida, J Koyama GEOPHYSICAL RESEARCH LETTERS 29- (9) 2002/05 [Not refereed][Not invited]

[1] From a temporal deployment of a broadband seismic network at Usu volcano, Japan, we observed long-period (12 sec) volcanic tremors during the first few weeks of the eruptive activity which started in the end of March, 2000. The source of these long period tremors are located relatively deep at a depth of 5 km, and their amplitude variation well correlates with the uplift rate of the eruption area. We thus attribute these long period tremors to the flow induced vibration of a magma chamber and its outlet located around the source region of the long period tremors. This may be the first seismological detection of long-period (> 10 sec) vibrations of a deep magma plumbing system. - 地震研究所報 77, 173-198- 2002 [Not refereed][Not invited]
**北海道日高衝突帯前縁部における屈折・広角反射法地震探査（大滝ー平取測線）** - 地震研究所報 77, 139-172- 2002 [Not refereed][Not invited]
**北海道日高衝突帯横断屈折・広角反射法地震探査（大滝ー浦幌測線）** - 月刊地球 24, 503-507- 2002 [Not refereed][Not invited]
**コーダ波振幅を用いた日高地方における３次元微細不均質分布のイメージング** - "Seismotectonics at the Subduction Zone", Terra Scientific Pub. Co, Tokyo 469-480- 2002 [Not refereed][Not invited]
**Seismic scattering from small-scale heterogeneities: Numerical Simulations and Observation** - T Mizuno, K Yomogida, H Ito, Y Kuwahara GEOPHYSICAL JOURNAL INTERNATIONAL 147- (3) 528 -542 2001/12 [Not refereed][Not invited]

In order to estimate the detailed orientation and density of cracks around an active fault area, we investigate the spatial distribution of shear wave anisotropy in the crust of the southern Hyogo region with high-quality waveform data recorded by a borehole network operated by the Geological Survey of Japan. We introduce the vertically aligned crack model of Hudson (1980, 1981) to explain its anisotropic structure. At stations more than 10 km away from the earthquake fault zone (i.e. Ikeda and Inagawa), crack orientation is nearly consistent with the direction of the regional maximum stress in this area (WNW-ESE). Cracks are distributed at depths below 5 km with a nearly constant crack density, epsilon, of 0.015 at Ikeda, and below 0 km with epsilon = 0.009 at Inagawa. These upper-limit depths of crack distribution are roughly comparable with those of microearthquakes there. In contrast, cracks are aligned nearly parallel to the strike of the earthquake fault (NE-SW) at stations at both ends of the fault system (i.e. Takarazuka and Ikuha), and their intensities of anisotropy are three or four times larger than those at Ikeda and Inagawa. At Takarazuka in particular, cracks are distributed at a depth shallower than 4.5 km with a large crack density, epsilon, of 0.06. This result suggests that major stress release by the faulting of the main shock dominates the present stress field at both ends of the fault system. In the middle of the fault system (i.e. Hirabayashi), cracks are aligned parallel to its regional maximum stress and the intensity of anisotropy is twice as large as those at Ikeda and Inagawa. This result contrasts with that of Tadokoro et al (1999), who determined the crack orientation at a station close to Hirabayashi to be parallel to the fault direction for events in a period earlier than this study by about a year, suggesting that the stress field in this area may have changed over the timescale of a year. Around Inagawa, the distribution of anisotropy seems to have strong azimuthal variations. This result indicates small-scale (of the order of several kilometres) variations of anisotropic structure in the upper crust, probably due to strong heterogeneities in the fluid and/or constitutive materials. - 北海道大学地球物理学研究報告 64- 81 -90 2001 [Not refereed][Not invited]
**2000年有株山噴火時の動測定「(共著)」** - 北海道大学地球物理学研究報告 64- 1 -19 2001 [Not refereed][Not invited]
**人工地震の広帯域地震観測による霸島火山群の浅部構造「(共著)」** - Kazunori Yoshizawa, Kiyoshi Yomogida, Seiji Tsuboi Geophysical Journal International 138- (1) 205 -220 1999/07 [Not refereed][Not invited]

The resolving power of polarization data compared with that of phase data is investigated by employing both synthetic and observed data sets, using the linear relationship between the phase velocity perturbation and the phase or polarization anomaly. In order to investigate the intrinsic differences between phase and polarization data, a synthetic test is first undertaken using a white noise model with sufficiently uniform coverage of ray paths. This test shows that polarization data can retrieve higher-order heterogeneities of degrees up to 20 almost completely, despite damping and smoothing effects, whilst phase data can only retrieve those of degrees lower than 8 with reasonable damping. Next, about 4000 phase and 2500 polarization records are collected for minor- and major-arc Rayleigh waves (R1 and R2) in the frequency range 4-12 m Hz. To correct the Rayleigh wave polarization data, the misorientation of each station is estimated from the polarization of long-period P waves propagating mainly in the lower mantle. The phase and polarization data are then inverted for the phase velocity distribution in spherical harmonics with degrees up to 15. The phase velocity maps derived from the phase data are quite consistent with previous studies, whilst those from the polarization data show some discrepancies. For example, the correlation between the phase and polarization models is quite good for low even degrees such as 2, 4 and 6, but not for low odd degrees or degrees higher than 8. The gradients of amplitude spectra from the polarization data are smaller than those from the phase data, especially at degrees higher than 6, which suggests a slightly higher sensitivity of the polarization data to higher-order heterogeneities. Nevertheless, the overall spectral characteristics of both models are similar that is, low-order heterogeneities are dominant whilst higher orders are clearly reduced. Further investigation using a synthetic test with the same uneven paths as the observed data shows the suppression of higher-order heterogeneities. Since the synthetic test with even paths retrieves higher-order heterogeneities sufficiently, this result strongly suggests the path averaging effect of uneven ray paths that is intrinsic in the ray theoretical approach used in this study as well as almost all the global inversions. Although inversions based on geometrical ray theory have some limited resolving power with the current status of global records, polarization data are indeed helpful in resolving higher-order lateral heterogeneities with the dense and uniform path coverage that is becoming available. - Journal of Faculty of Science, Hokkaido University Series (]G0007[) 11- 611 -632 1999 [Not refereed][Not invited]
**Synthetic seismograms near a finite fault system「(共著)」** - K Yomogida, R Benites, R Robinson EARTH PLANETS AND SPACE 50- (4) 303 -312 1998 [Not refereed][Not invited]

Using an indirect boundary integral method, seismograms are computed for elastic media with localized heterogeneities. Models are two-dimensional homogeneous full spaces (P and SV waves) with many circular cavities as heterogeneities or scatterers. Heterogeneities are localized within a depth range, forming a relatively thin random layer. Seismograms are obtained for receivers at depth 0 km (surface) and for several focal depths, with sources radiating S waves isotropically. Seismograms are composed of the direct S wave and all the possible scattered waves by the heterogeneities, exhibiting late arrivals or coda waves. The coda wave amplitude, or coda energy level, and its duration vary for events with different focal depths. As the focal depth increases and the source gets closer to the layer of localized heterogeneities, the coda level becomes small. When the source is within the heterogeneous layer, however, the coda level becomes larger than for a case of the source either above or below the heterogeneous layer. This local enhancement of coda takes place clearly only in the frequency range for which the scattering is the most effective, that is, when the non-dimensional frequency kd takes values from 2 to 3, where k is the wavenumber and d is the size of each heterogeneity. Such enhancement of coda is not observed when the density of cavities, or strength of the heterogeneities, is reduced. Coda level becomes locally large for a source within the heterogeneous layer only in the case that the heterogeneities are strong enough to excite multiply scattered waves, as compared with the singly scattered ones. Robinson (1987) studied the temporal variation of coda-duration magnitude relative to event magnitude based on P and S wave amplitudes using the seismic network of the Wellington, New Zealand, region. He found that coda-duration magnitude relative to amplitude magnitude decreases with focal depth and becomes large locally for events in the depth range from 65 and 75 km. Our synthetic seismograms explain his results well, implying that there must be a region of localized, strong heterogeneity at depths around 70 km. The effective size of the heterogeneous region may be of about several kilometers because the observation was performed with 1 Hz seismometers. This localized heterogeneous layer is probably associated with the subducting Pacific plate underneath the Wellington region. - K Yomogida, R Benites, PM Roberts, F Fehler PHYSICS OF THE EARTH AND PLANETARY INTERIORS 104- (1-3) 175 -192 1997/11 [Not refereed][Not invited]

The boundary integral representation of the complete seismic wavefield in two-dimensional composite media characterised by the distribution of many cavities (Paper I in this issue) is used to study the waveforms and spectra of the scattered wavefield for three models of media heterogeneity, upon the incidence of P and S plane waves and line sources. First, the case of one circular cavity and S primary waves shows that the scattered wavefield is composed mainly of S waves, and that S-P scattering can be ignored in any frequency ranee for all forward scattering angles (scattering angle theta measured clockwise with respect to the direction of propagation of the primary wave). The spectra of forward scattering computed for theta similar or equal to 0 degrees resemble the spectrum predicted by the Born approximation for acoustic or scalar waves for theta = 0 degrees: of small amplitudes for small values of non-dimensional frequency kd (k is the wavenumber and d is the cavity diameter), increasing with kd, up to kd similar or equal to 2, and becoming constant for larger values of kd. The spectra for backward scattering (theta similar or equal to 180 degrees) behave similarly, showing amplitudes as large as those computed for the forward cases. The non-isotropic pattern of scattering predicted by analytical solutions is also confirmed. In the case of P primary waves, P-S scattering appears to be significantly stronger than P-P scattering for most scattering angles, except for theta similar or equal to 0 degrees and theta similar or equal to 180 degrees. The computation of synthetic seismograms for models with many cavities show scattered waves of low frequency corresponding to wavelengths much larger than the size of the cavities, as well as those of high frequency due to multiple reflections and conversions at the boundaries of the cavities. A cluster of 20 cavities randomly distributed within a small region produces well-defined low frequency waves that appear to be associated with the presence of one low-velocity heterogeneous body, or soft inclusion, represented by the whole cluster. The case of 50 cavities randomly distributed within a horizontally extended region (of narrow thickness) shows coda-like wave arrivals, particularly strong in the horizontal component. Also in this case, nearly horizontally incident plane waves produce low frequency scattered waves of large amplitudes. It appears that while in the long-wavelength limit this model synthesises a coherent wave corresponding to reflection upon a horizontal interface, towards the short-wavelength limit the scattered waves show a rather complex, incoherent pattern immediately after the arrival of the incident wave, as if the region were a transitional zone of effective thickness. The analysis presented in this paper suggests that if the wavelengths are much larger than the size of the cavities, our representation of random media can he used to represent regional heterogeneity in the earth's crust, associated with observed seismic scattering phenomena. (C) 1997 Elsevier Science B.V. - K Tsuruga, K Yomogida, S Honda, H Ito, T Ohminato, H Kawakatsu JOURNAL OF VOLCANOLOGY AND GEOTHERMAL RESEARCH 75- (3-4) 337 -358 1997/02 [Not refereed][Not invited]

We conducted broadband seismic observations at three sites of the Sakurajima Volcano, Japan, from December 1992 to March 1993 in order to clarify the spatial and temporal variations of spectral properties of volcanic earthquakes: B-type earthquakes and volcanic tremor episodes. We used three STS-2 seismometers recording in the frequency range from 0.03 to 6 Hz. Major spectral peaks of both B-type earthquakes and volcanic tremor episodes are located in the frequency range of 1.1-1.3, 2.3-2.5 and 3.4-3.6 Hz. From the similarities of temporal variations in spectra among all the stations and of body-wave propagation in an early part of seismograms, spectral peaks of B-type earthquakes and volcanic tremor episodes mainly reflect source characteristics rather than site and path effects. Temporal variations of the dominant peaks are classified into three types for B-type earthquakes and into four types for volcanic tremor episodes. Two types of B-type earthquakes are similar to two of volcanic tremor episodes in terms of temporal variations of spectral peaks and of polarization characteristics with the only difference in amounts of energy. The similarities in the temporal variation of spectral properties indicate that B-type earthquakes and volcanic tremor episodes share common source mechanisms with different energy magnitudes. Since volcanic tremor episodes tend to take place prior to the swarms of B-type earthquakes and explosion earthquakes associated with any summit eruptions, we conclude that volcanic tremor episodes occur at the early stage of low-energy radiation, followed by B-type earthquakes as higher energy is radiated. - K Yomogida, K Aki, R Benites GEOPHYSICAL JOURNAL INTERNATIONAL 128- (2) 425 -433 1997/02 [Not refereed][Not invited]

Using the indirect boundary integral scheme for multiple scattering of seismic waves developed by Benites, Aki & Yomogida (1992), we compute SH-wave seismograms and measure frequency-dependent characters of coda Q in 2-D random media with a hat layer over a half-space. Many circular cavities are randomly distributed in both the upper layer and the half-space, down to a certain depth (called the lower layer), simulating the upper and lower crusts. The scattering strength and the intrinsic attenuation, Q(i) are varied for each layer, and the S-wave velocity is prescribed to be constant throughout the medium so that the computation of Green's functions for the boundary integral is simple. Considering two basic parameters of our random media, scattering strength and intrinsic attenuation Q(i), we represent the shallow-earth structure by an upper crust with large intrinsic attenuation and a lower crust with effective scatterers. Computations of coda Q for several values of those parameters show that when the scattering is relatively strong, coda Q(-1) is roughly independent of frequency. This result differs from the case of a uniformly random model where coda Q(-1) peaks around kd = 2, where k is the wavenumber and d is the cavity diameter. If the scattering strength in the lower layer is large enough for multiple scattering to dominate over single scattering, coda Q(-1) strongly depends on the intrinsic attenuation in the lower layer, Q(i2)(-1), and these two values (coda Q(-1) and Q(i2)(-1)) become similar. We explain this feature as follows. Waves scattered in the upper layer attenuate quickly due to high intrinsic attenuation and contribute little to the coda envelope in a time window starting at twice the traveltime of the direct wave. Multiply scattered waves in the lower layer eventually arrive at the surface, dominating the coda envelope, which decays at a rate determined by the intrinsic attenuation in the lower layer, Q(i2)(-1). The hypothesis that the temporal decay of coda is controlled not by the scattering but by the energy leakage into a 'transparent' underlying mantle is ruled out in general by our numerical simulations, except at low frequencies. Although our model may be too simple to simulate the details of observed coda Q, coda Q is likely to reflect the intrinsic attenuation in the Earth's lower crust. - Physics of Earth and Planetary Interiors 103- 1 -16 1997 [Not refereed][Not invited]
- Kiyoshi Yomogida, Takashi Nakata Journal of Physics of the Earth 45- (2) 155 -165 1997 [Not refereed][Not invited]
- K Yomogida, R Benites PURE AND APPLIED GEOPHYSICS 148- (1-2) 255 -268 1996/11 [Not refereed][Not invited]

Numerical modelling of SH wave seismograms in media whose material properties are prescribed by a random distribution of many perfectly elastic cavities and by intrinsic absorption of seismic energy (anelasticity) demonstrates that the main characteristics of the coda waves, namely amplitude decay and duration, are well described by singly scattered waves in anelastic media rather than by multiply scattered waves in either elastic or anelastic media. We use the Boundary Integral scheme developed by BENITES et nl. (1992) to compute the complete wave field and measure the values of the direct wave Q and coda waves Q in a wide range of frequencies, determining the spatial decay of the direct wave log-amplitude relation and the temporal decay of the coda envelope, respectively. The effects of both intrinsic absorption and pure scattering on the overall attenuation can be quantified separately by computing the Q values for corresponding models with (anelastic) and without (elastic) absorption. For the models considered in this study, the values of coda Q(-1) in anelastic media are in good agreement with the sum of the corresponding scattering Q(-1) and intrinsic Q(-1) values, as established by the single-scattering model of AKI and CHOUET (1975). Also, for the same random model with intrinsic absorption it appears that the singly scattered waves propagate without significant loss of energy as compared with the multiply scattered waves, which are strongly affected by absorption, suggesting its dominant role in the attenuation of coda waves. - Vavrycuk, V, K Yomogida WAVE MOTION 23- (1) 83 -93 1996/02 [Not refereed][Not invited]

Tensor equations of the ray theory for homogeneous anisotropic elastic media are presented. For a point source, an explicit solution of the transport equation is obtained, thus the additional as well as principal components of the ray amplitudes for higher-order ray approximations are expressed only by differential operators of lower-order terms. Possibility of analytical calculation of higher-order approximations is exemplified for SH waves in a transversely isotropic medium. The ray series of the SH-wave Green tensor for the transversely isotropic medium involves only two non-zero terms and the complete ray solution coincides with an exact solution obtained by other complicated procedures. - Tono, Y., Yomogida, K. Journal of Physics of the Earth 44- (6) 729 -744 1996 [Not refereed][Not invited]
- K Yomogida JOURNAL OF PHYSICS OF THE EARTH 44- (6) 655 -656 1996 [Not refereed][Not invited]
- K YOMOGIDA, R BENITES GEOPHYSICAL JOURNAL INTERNATIONAL 123- (2) 471 -483 1995/11 [Not refereed][Not invited]

Using the boundary integral method to simulate SH waves numerically in 2-D homogeneous full- or half-space media with randomly distributed cavities, we compare the amplitude attenuation of direct waves with the temporal decay of the coda. The boundary integral method includes the effect of any degree of multiply scattered waves for a wide frequency range, up to wavelengths smaller than the size of the cavities. We consider seismograms on the free surface so that heterogeneities exist only on one side of the receivers, a situation that resembles actual seismic observations. Seismograms are computed for a vertically incident plane wave and for an isotropic line source. In both cases, the value of Q(-1) as a function of kd, where k is the wavenumber and d is the cavity diameter, peaks around kd=2 for the direct wave, which is consistent with some single-scattering models. Coda Q(-1) determined by the temporal decay of the coda envelope agrees well with Q(-1) for the direct wave for models with a root-mean-square fluctuation of velocity, sigma, of about 10 per cent in a half-space. On the other hand, the coda Q(-1) is systematically larger than the direct wave Q(-1) in full-space models, that is, without the inclusion of the reflection at the free surface. When the cavity density is doubled (sigma>20 per cent), the coda energy increases rapidly and its temporal decay decreases, so that coda Q(-1) becomes smaller than the direct wave Q(-1), even for full-space models. With a smaller value of sigma (about 5 per cent), the coda decays rapidly and the relation between the two types of Q(-1) is reversed: the coda Q(-1) becomes larger than the direct wave Q(-1). By comparing results from seismograms composed only of singly scattered waves with those that include multiply scattered waves, we can compare the relative contribution of each singly and multiply scattered wavefield to the two measures of Q. Single scattering mainly determines both the direct wave Q(-1) and the coda Q(-1) for the smallest value of sigma, while the values of both kinds of attenuation, particularly the direct wave Q(-1), are strongly affected by multiple scattering when sigma is large. Our results imply that a reasonable estimate of scattering attenuation can be obtained by measuring the temporal decay of the coda, if the scattering character of the Earth is similar to our models with a sigma of around 10 per cent, where the single scattering is found to be dominant. - VAVRYCUK, V, K YOMOGIDA GEOPHYSICAL JOURNAL INTERNATIONAL 121- (3) 925 -932 1995/06 [Not refereed][Not invited]

An analytical approach to the calculation of higher-order ray approximations for wavefields generated by point sources in homogeneous isotropic media is presented. It is shown that the near-field waves neglected by the zeroth-order ray approximation can be incorporated by considering higher-order terms of the ray series. Assuming the point source to be a unit single force, we calculate the complete ray-theoretical Green's function. The ray series of the Green's function consists only of three non-zero terms and the ray solution coincides with an exact solution, Wavefields radiated by general multipolar sources are also exactly expressed by the ray theory. Simple algebraic formulae for the spherical harmonics coefficients of all higher-order ray approximations are established. - 中田 高, 蓬田 清, 尾高 潤一郎, 坂本 晃章, 朝日 克彦, 千田 昇 地学雑誌 104- (1) 127 -142 1995 [Not refereed][Not invited]
- Journal of Natural Disaster Science 16- (3) 11 -19 1995 [Not refereed][Not invited]
**Relation between seismic observation and the fault complexity of the 1995 hyogoken-nambu earthquake** - Journal of Natural Disaster Science 16- (3) 1 -9 1995 [Not refereed][Not invited]
**Surface funlt characteristics of the 1995 Hyogoken-nambu earthquake** - Takashi NAKATA, Kiyoshi YOMOGIDA, Jun-Ichiro ODAKA, Teruaki SAKAMOTO, Katsuhiko ASAHI, Noboru CHIDA Journal of Geography 104- (1) 127 -142 1995 [Not refereed][Not invited]
- Journal of Facutay of Scicnce, Hiroshima University C11- 1 -9 1995 [Not refereed][Not invited]
**Multi-site Broadband seismic observation at Sakurajima Volcano, Japan** - K YOMOGIDA, T NAKATA GEOPHYSICAL RESEARCH LETTERS 21- (17) 1799 -1802 1994/08 [Not refereed][Not invited]

We found more than fifty people who provided important information on coseismic fault motion of the 1990 Luzon, the Philippines, earthquake. About thirty of them ''witnessed'' coseismic fault motion close to (less than 30 m) the surface rupture. The average slip duration reported was about 1 s, and some reported much less than 1 s, which is surprisingly short compared with the predicted value of about 10 s for its size (e.g., fault length: 120 km, slip: 5 m) according to scaling laws obtained from teleseismic studies. Teleseismic studies only obtain spatially and temporally averaged values of fault motion, but actual coseismic fault slip has a high degree of spatial heterogeneity. The large slip velocity over 10 m/s obtained by our study suggests a very high local stress drop (> 100 MPa). Other important information from the eyewitness accounts is that the slip velocity seems to have been nearly constant, implying that the fault slip is well described by a ramp function proposed by Haskell. - Journal of Faculty of Science, Hiroshima University C10- 1 -9 1994 [Not refereed][Not invited]
**Observation of micro tremers in Tsukuba aea, Japan, by a broad-band seismometer** - K YOMOGIDA GEOPHYSICAL JOURNAL INTERNATIONAL 116- (1) 119 -130 1994/01 [Not refereed][Not invited]

Strong motion data at La Union, one of three near-fault stations of the Michoacan, Mexico, earthquake in 1985, are analysed by a wavelet transform in order to identify a clear later energy arrival whose frequency content and particle motion are quite different from the main part of the seismograms. We used an orthonormal set of analysing wavelets, a discrete wavelet transform, proposed by Meyer and Yamada, with which an efficient computational procedure can be achieved utilizing the fast Fourier transform. Results of an application of the above wavelet transform to three-component velocity data of La Union are summarized as follows. A vertical seismogram does not contain any distinguishable later energy arrivals for the entire frequency range recorded. The major energy arrives in the time interval between 10 and 30 s. In contrast, an east-west component seismogram shows a peculiar later arrival more than 10 s later than the main energy arrival, and our wavelet analysis shows that this later phase is characterized by an arrival time of 38 s and a frequency range from 0.1 to 1 Hz where the amplitude of the later phase is even larger than the main part. This example clearly demonstrates the potential of wavelet transforms to identify objectively any phase in seismograms localized in both time and frequency. From the information on its particle motion, the above phase corresponds to a heterogeneous and delayed break of one stronger portion on a fault on which the rupture was very smooth or 'crack-like' elsewhere. - K YOMOGIDA, JT ETGEN BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 83- (5) 1325 -1344 1993/10 [Not refereed][Not invited]

Elastic wave propagation within and around the Los Angeles basin during the Whittier-Narrows earthquake in 1988 is simulated with a 3-D finite difference method. The model consists mainly of a sedimentary layer over the basement, but lateral variations inside the sedimentary basin is also included to approximate observed variations in surface shear velocity. The minimum shear velocity is 1 km/sec on the surface and fairly reliable results are obtained up to 0.75 Hz. The total simulation lasts 26 sec. Our simulation can reproduce the general amplification pattern: small east of the epicenter and large in the L.A. basin and the San Fernando Valley, due to thick sediments of low velocity in these areas. Surface waves are generated by the primary S wave and trapped inside the basins. In the southern part of the L.A. basin, a distinct later phase, delayed by 5 to 6 sec relative to the primary S wave, is produced in our simulation, which matches some strong motion records. - H KAWAKATSU, T OHMINATO, H ITO, Y KUWAHARA, T KATO, K TSURUGA, S HONDA, K YOMOGIDA GEOPHYSICAL RESEARCH LETTERS 19- (19) 1959 -1962 1992/10 [Not refereed][Not invited]

We installed a portable broadband seismometer (Streckeisen STS-2) at the Sakurajima volcano, which has been very active in the recent years. The recorded seismograms show a wide variety (both in temporal and spectral contents) of seismic events, from explosions to tremors, and exhibit the importance of such broadband seismometry at volcanos. We present examples of seismograms to show the potential of broadband seismic observation in monitoring volcanic activities. - N MATSUMOTO, K YOMOGIDA, S HONDA GEOPHYSICAL RESEARCH LETTERS 19- (4) 357 -360 1992/02 [Not refereed][Not invited]

Complexity of fault systems in Japan and the Philippines is quantitatively measured using fractal dimension. The areas studied are the Median Tectonic Line, Izu, Unzen on the Japanese Islands, and the fault system yielded by the Philippine earthquake which occurred on July 16, 1990. Results are summarized as follows: Fractal dimensions along the Median Tectonic Line vary between 1 and 1.3. The Unzen area gives the highest fractal dimension (1.4) of all the areas studied, which implies that the fault system there is the most complex. Fractal dimension of the fault system associated with the Philippine earthquake is smaller than 1. It can be interpreted that this fault system is almost one-dimensional, with truncated by numerous gaps of various sizes. - SD RUPPERT, K YOMOGIDA PURE AND APPLIED GEOPHYSICS 138- (3) 407 -427 1992 [Not refereed][Not invited]

Evidence supporting a smooth crack-like rupture process of the Michoacan earthquake of 1985 is obtained from a major earthquake for the first time. Digital strong motion data from three stations (Caleta de Campos, La Villita, and La Union), recording near-field radiation from the fault, show unusually simple ramped displacements and permanent offsets previously only seen in theoretical models. The recording of low frequency (0 to 1 Hz) near-field waves together with the apparently smooth rupture favors a crack-like model to a step or Haskell-type dislocation model under the constraint of the slip distribution obtained by previous studies. A crack-like rupture, characterized by an approximated dynamic slip function and systematic decrease in slip duration away from the point of rupture nucleation, produces the best fit to the simple ramped displacements observed. Spatially varying rupture duration controls several important aspects of the synthetic seismograms, including the variation in displacement rise times between components of motion observed at Caleta de Campos. Ground motion observed at Caleta de Campos can be explained remarkably well with a smoothly propagating crack model. However, data from La Villita and La Union suggest a more complex rupture process than the simple crack-like model for the south-eastern portion of the fault. - K YOMOGIDA PURE AND APPLIED GEOPHYSICS 138- (3) 391 -406 1992 [Not refereed][Not invited]

We propose a different kind of seismic inversion from travel-time or waveform inversion for lateral heterogeneities in the earth: Fresnel zone inversion. Amplitude and phase delay of data in several frequency ranges are inverted for model space around ray paths with a width corresponding to the considered frequency so that primary effect of finiteness of wavelength be included. For vertically heterogeneous media, Frechet derivatives for inversion are obtained very efficiently using the paraxial ray approximation, with nearly similar amounts of computation compared to travel-time inversion. As an example, Frechet derivatives are computed for a teleseismic observation system for a three-dimensional structure in the lithosphere beneath an array of seismic stations. Even if the used frequency is around 2 Hz, the width of Frechet derivatives cannot be neglected, particularly near the bottom of the lithosphere. Sensitivity of model parameters to observations is, moreover, different in our approach from conventional travel-time inversion: it is zero along ray paths but large slightly away from them. Some model calculations show that travel-time inversion, particularly with models divided into very fine meshes or blocks, might give misleading results. An example of inversion for a simple Camembert model, in the event that travel-time inversion gives no reliable results, shows how this technique works with much smaller data sets and computation than waveform inversions. - R BENITES, K AKI, K YOMOGIDA PURE AND APPLIED GEOPHYSICS 138- (3) 353 -390 1992 [Not refereed][Not invited]

The full waveform synthetic seismogram of multiple scattered SH waves by many cylindrical cavities in two-dimensional homogeneous elastic media is computed. We used the so-called "single-layer potential" integral representation of the scattered field and a discretization scheme with line source distribution for each cavity. The total field is the sum of the incident wave plus the field radiated from all sources, each multiplied by an unknown complex constant representing its strength. These constants are determined by imposing the appropriate boundary conditions in the least-squares sense. Here we solve scattering problems involving one, two, four, twelve and fifty cavities regularly distributed in a half-space. The seismograms computed along the free-surface show regions where the incident wave is strongly attenuated, as well as the arrivals of all multiple scattered phases. The accuray of the method is estimated from the degree of agreement of our solution for one cavity with the corresponding analytical solution, and also from the magnitude of the residual tractions along the boundaries of two cavities separated at various distances. Finally we apply the method to compute the case of fifty cylindrical cavities, each of radius a, randomly distributed in a region 80a wide by 30a deep in a half-space. The value of scattering loss is obtained from the amplitude decay of the primary wave with distance for wavelengths in the range from 1.7a to 13.3a, using the synthetic seismogram calculated for the same distribution of 50 cavities as above, but in full-space. - S HONDA, K YOMOGIDA GEOPHYSICAL RESEARCH LETTERS 18- (8) 1381 -1384 1991/08 [Not refereed][Not invited]

An earthquake which occurred off the northern Mozambique gives a peculiar feature of P-waves recorded by WWSSN long period instruments. All the P-waveforms can be divided into two parts: the amplitude of the first arriving phase varies from station to station, which is consistent with the normal fault type solution of the Harvard CMT solution determined by the long period GDSN data. The amplitude of the second phase, in contrast, is fairly constant over all the stations, inconsistent with the normal fault solution. From these observations and data analyses, we propose that the source process of this event is a combination of a normal fault and a subsequent isotropic source starting with an explosion followed by a contraction or a corruption with each net moment of 2 approximately 3 x 10(25) dyne.cm at the assumed depth of 7.5 km. Our image of these source mechanisms is consistent with the known geological setting around the epicenter: grabens with many dikes or intrusions. - 地震 42- 1989 [Not refereed][Not invited]
**横方向不均質媒体における波動場の最新の計算法** - K YOMOGIDA GEOPHYSICAL RESEARCH LETTERS 15- (11) 1223 -1226 1988/10 [Not refereed][Not invited]
- K YOMOGIDA GEOPHYSICAL JOURNAL OF THE ROYAL ASTRONOMICAL SOCIETY 88- (1) 297 -304 1987/01 [Not refereed][Not invited]
- K YOMOGIDA, K AKI GEOPHYSICAL JOURNAL OF THE ROYAL ASTRONOMICAL SOCIETY 88- (1) 161 -204 1987/01 [Not refereed][Not invited]
- K YOMOGIDA, K AKI JOURNAL OF GEOPHYSICAL RESEARCH-SOLID EARTH AND PLANETS 90- (NB9) 7665 -7688 1985 [Not refereed][Not invited]
- K YOMOGIDA GEOPHYSICAL JOURNAL OF THE ROYAL ASTRONOMICAL SOCIETY 82- (3) 511 -533 1985 [Not refereed][Not invited]
- K YOMOGIDA, T MATSUI EARTH AND PLANETARY SCIENCE LETTERS 68- (1) 34 -42 1984 [Not refereed][Not invited]
- K YOMOGIDA, T MATSUI JOURNAL OF GEOPHYSICAL RESEARCH 88- (NB11) 9513 -9533 1983 [Not refereed][Not invited]

**火山性地震の動的挙動**その他の研究制度**大地震時の断層の動的挙動**その他の研究制度**強い不均質媒体での地震波の伝幡**科学研究費補助金**Dynamic source process of volcanic earthquakes**The Other Research Programs**Dynamic behaviors of fault motions associated with a large earthquake**The Other Research Programs**Seismic wave propagation in strongly heterogeneous media**Grant-in-Aid for Scientific Research

- Internal Structure of the Earth開講年度 : 2018課程区分 : 修士課程開講学部 : 理学院キーワード : 理論地震学、内部構造
- Inter-Graduate School Classes(General Subject):Natural and Applied Sciences開講年度 : 2018課程区分 : 修士課程開講学部 : 大学院共通科目
- Special Lecture II開講年度 : 2018課程区分 : 修士課程開講学部 : 理学院キーワード : reflection seismology, physical properties of rocks, wireline logging, stress, structure, geomechanics, logging correlation,
- Special Lecture II開講年度 : 2018課程区分 : 博士後期課程開講学部 : 理学院キーワード : reflection seismology, physical properties of rocks, wireline logging, stress, structure, geomechanics, logging correlation,
- Continuum Dynamics for Earth Sciences開講年度 : 2018課程区分 : 学士課程開講学部 : 理学部キーワード : 連続体、弾性、粘性流体、応力とひずみ、定量的モデル、地球惑星科学の諸問題
- Basic Earth and Planetary Science Ⅰ開講年度 : 2018課程区分 : 学士課程開講学部 : 全学教育キーワード : 固体地球，地球の構造と構成物質，地球史，変動する地球，自然災害