progress report

PROGRESS REPORT 2003-2006

Task Group on Scattering and Heterogeneity IASPEI Commission on Seismological Observation and Interpretation

June 30, 2006

Haruo SATO Chair of the task group Dept. of Geophysics, Graduate School of Science, Tohoku University Aramaki-Aza-Aoba 6-3, Aoba-ku, Sendai-shi, Miyagi-ken, 980-8578, Japan Phone: 81-22-795-6531, Fax: 81-22-795-6783 E-mail: [email protected]

Progress Report of the Task Group on Scattering and Heterogeneity for 2003-2006

The objectives of the Task Group are to promote the international collaborations and interactions among scientists around the world on wave propagation and scattering in heterogeneous earth. The specific tasks of the Task Group could include: Organize international meetings, workshops, training courses on the related topics; Exchange information in an international scope periodically through internet and e-mails; Compile progress reports on wave propagation and scattering in heterogeneous media, in which the theoretical and methodological progresses during the period should be summarized; Sponsor and organize special programs; Maintain a constructive relation with AGU, EGU, SEG, EAGE, SSA, SSJ, SEGJ and other regional, professional associations by co-sponsoring some symposia, workshops and training courses of common interests.

1. Members of the second term The second term started at the general assembly in Santiago. Followings are the members of the second term and Haruo SATO serves as the chair of the task group. "Prof. Michel Campillo" Laboratory of Geophysics and Tectonophysics of Grenoble, BP 53X, Grenoble 38041, France "Prof. Xiaofei Chen" Department of Geophysics, Peking University, Beijing 100871, P. R. China "Prof. Edoardo Del Pezzo" INGV- Osservatorio Vesuviano, via Diocleziano, 328, 80124 Napoli, Italy "Dr. Mike Fehler" Los Alamos National Laboratory, MS D443, Los Alamos, NM 87545, USA "Dr. John Goff" Institute for Geophysics, University of Texas, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759, USA "Dr. Alexander A. Gusev" Institute of Volcanic Geol. and Geochemistry, Piip Blvd. 9, Petropavlovsk-Kamchatsky 683006, Russia "Prof. Klaus Holliger" Applied and Environmental Geophysics, Institute of Geophysics, University of Lausanne, CH-1015 Lausanne, Switzerland "Dr. Ludek Klimes" Department of Geophysics, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 3, 121-16 Praha 2, Czech Republic "Prof. Michael Korn" Institute for Geophysics and Geology, University of Leipzig, Talstrasse 35, D-04103 Leipzig, Germany "Prof. Yu Kravtsov" Institute of Mathematics, Physics and Chemistry, Maritime University in Szczecin, Waly Chrobrego 1/2, 70-500 Szczecin, Poland "Prof. Alan Levander" Geol & Geohys Dept, Rice University, MS 126, 6100 Main St., Houston, TX 77005, USA "Dr. Ludovic Margerin" Laboratory of Geophysics and Tectonophysics of Grenoble, BP 53X, Grenoble 38041, France "Dr. Satoshi Matsumoto" Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shin'yama 2,Shimabara-shi, 855-0843, Japan "Dr. Valerie Maupin" University Oslo, Dept Geology, POB 1047, Oslo, 0316, Norway "Dr. Kevin Mayeda" Ground Based Nuclear Explosion Monitoring

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Program, Non-Proliferation, Arms Control & International Security Division, Lawrence Livermore National Laboratory, PO Box 808, L-428 Livermore, CA 94551, USA "Dr. Osamu Nishizawa" AIST Tsukuba Central, Tsukuba-shi, Ibaraki-ken 305-8561, Japan "Dr. W. Scott Phillips" Los Alamos National Laboratory, MS D443, Los Alamos, NM 87545, USA "Prof. Haruo Sato" Dept. of Geophysics, Graduate School of Science, Tohoku University, Aoba-ku, Sendai-shi 980-8578, Japan "Prof. Serge Shapiro" Fachrichtung Geophysik, Freie Universitaet Berlin, Malteserstrasse 74-100, Build.D., D-12249 Berlin, Germany "Dr. Ru-Shan Wu" Center for the Study of Imaging and Dynamics of the Earth, Institute of Geophysics and Planetary Physics, University of California, Santa Cruz, CA 95064, USA

2. Publications -First volumeAs a scientific product of the first term chaired by R. S. Wu, a book entitled “Advances in Wave Propagation in Heterogeneous Earth” (Eds. Wu and Maupin) has been completed as a volume of Advances in Geophysics Series of Elsevier Publishing (Series editor R. Dmowska). It is now in press and will appear this summer. Chapter titles are as follows: 1. Cerveny, V., L. Klimes and I. Psencik: Seismic Ray Method: Recent Developments 2. Maupin V.: Introduction to Mode coupling methods for surface waves 3. M. Bouchon and F. Sanchez-Sesma: Boundary Integral Equations and Boundary Element Methods in Elastodynamics 4. X. F. Chen: Generation and propagation of seismic SH waves in multilayered media with irregular interfaces 5. R. S. Wu, X. B. Xie and X. Y. Wu: One-way and one-return approximations for fast elastic wave modeling in complex media 6. R. S. Wu, X.Y. Wu and X. B. Xie: Simulation of high-frequency wave propagation in complex crustal waveguides using generalized screen propagators 7. Chaljub, E., D. Komatitsch, J. Vilotte, Y. Capdeville, B Valette and G. Festa: Spectral element analysis in seismology 8. Moczo, P., J. Robertsson and L. Eisner: The finite-difference time-domain method for modeling of seismic wave propagation 9. L. Huang: A Lattice-Boltzmann Approach to Acoustic Wave Propagation 10. H. Sato and M. Fehler: Synthesis of Scalar-Wave Evnvelopes in 2-D Random Media -Second volumeHaving received individual progress reports from active members and extended members of the task group (see section 5), we have started to edit a book entitled "Scattering of Short-Period Seismic Waves in Earth Heterogeneity". H. Sato and M. Fehler serve as editors of this book consists of 15 chapters.

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3. Workshops and sessions After the general assembly of IASPEI in Santiago in 2005, we have organized following workshops and sessions having the same objectives of our Task Group: * Workshop on “Interpretation of physical properties of small scale heterogeneity in the lithosphere” (Convener J. Kawahara), Earthquake Research Institute, Univ. Tokyo, Tokyo, Japan, 12-13 Jan. 2006. Proceedings (in Japanese) are available from http://wwweic.eri.u-tokyo.ac.jp/viewdoc/ * Session on "Multiple Scattering, Random Wave Fields, and Dissipation" (Conveners: U. Wegler, M. Korn, and L. Margerin) was held at EGU General Assembly, Vienna, 2-7 April, 2006. * Session on “Seismic wave propagation” (Conveners Nishizawa, O., H. Sato, H. Mikada, T. Matsuoka and T. Watanabe) and a Session on “Memorial session for late Kei Aki” (Conveners Yomogida, K., Sato, N. Hirata, M. Ukawa, and H. Kawase), Japan Geoscience Union Meeting, Makuhari, Japan, 14-18 May 2006.

4. Web site “Scattering and Heterogeneity in the Earth” For exchanging scientific information about seismic wave scattering in the heterogeneous earth medium, we have maintained a special web site of our task group since 2001: http://www.scat.geophys.tohoku.ac.jp/. News about workshops, recent publications, and key references are updated.

5. Reports of scientific activities This progress report is a compilation of reports of members and active extended members of the task group as a state of the art summary for developments of studies on seismic wave propagation through earth medium heterogeneity since IUGG 2003 in Sapporo, Japan. A pdf file of this progress report “ProgRep2006.pdf” can be downloaded from the web site of this task group: http://www.scat.geophys.tohoku.ac.jp/. We hope it will be useful for mutual understanding of studies developed in various people in the world. -Theoretical works and experimental worksFrancisco J. Sánchez-Sesma reported to deal with the Green function retrieval from correlations in the elastic 2D and 3D cases. Ayse Kasiliar, S. Shapiro, S. Buske and Yu. A. Kravtsov reported inverse scattering of surface waves and the analysis of travel time fluctuations in random media by using geometrical optics method. Kawahara reported envelope synthesis of SH waves in a crack distributed 2D medium based on numerical simulation. Michael Korn, Jens Pszbilla and Ulrich Wegler reported the envelope synthesis of vector waves in random media based on the radiative transfer theory with the Born approximation. Ulrich Wegler reported diffusion of wave energy in layered scattering media. Haruo Sato and Michael Korn reported the envelope synthesis of vector waves in random elastic media based on the Markov approximation for the parabolic equation. Tatsuhiko Saito reported the envelope synthesis of scalar waves in nonisotropic random media. Tobias Müller and Serge A. Shapiro reported scattering characteristics of seismic primary wave fields in 2-D and 3-D random media. Tobias Müller and Boris Gurevich reported attenuation and dispersion of seismic waves due to wave-induced flow in random poroelastic media. Osamu Nishimura

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and Genshiro Kitagawa reported phase fluctuation in random media revealed by laboratory rock experiments by using laser Doppler vibrometers. -Observational works in regional scaleHisashi Nakahara reported envelope inversion studies for high-frequency seismic energy radiation from earthquake faults. Separation of attenuation into intrinsic and scattering loss characteristics was reported by Niren Biswas in Alaska and by Edoardo Del Pezzo at volcanoes in Italy, by Aranza Ugalde in western India. Kazuo Yoshimoto reported spatial distribution of S-coda-wave energy and seismic attenuation structure in Japan. Tsutomu Takahashi reported the path dependence of broadening of S-wave envelopes in Honshu, Japan. He found strong heterogeneity beneath Quaternary volcanoes and transparency between them. Satoshi Matsuoto developed a new method for inverting the array seismic data for the spatial distribution of S-wave scattering coefficient. Aranza Ugalde, E. Carcolé, and J. N. Tripathi reported coda envelope inversion for imaging the distribution of scatterers in the crust in southern India. Yoshio Murai reported the determination of a scatterer location by using an ocean bottom seismograph and controlled sources. Masatoshi Miyazawa detected triggering effect of earthquakes caused by a passage of seismic waves. Ulrich Wegler reported temporal change in velocity in volcanoes based on the coda interferometry. -Observational works in global scaleWon Sang Lee measured scattering coefficient in the mantle from the coda envelope analysis before and after ScS arrivals. Won Sang Lee found that a power law decay is more appropriate for explaining coda envelope decay for period from 1-10s in a long lapse time window. Takuto Maeda reported the dominance of higher modes for a period band of 100-200 sec at long lapse time as 20 hours from the origin time. Contributing authors of this progress reports are N. Biswas, E. Del Pezzo, A. Kaslilar, J. Kawahara, M. Korn, Yu. A. Kravtsov, W. S. Lee, T. Maeda, S. Matsumoto, M. Miyazawa, T. M. Müller, Y. Murai, H. Nakahara, O. Nishizawa, J. Przybilla, T. Saito, F. J. Sánchez-Sesma, H. Sato, T. Takahashi, A. Ugalde, U. Wegler and K. Yoshimoto. The reports are in alphabetical order. We would like to express sincere thanks to all the contributing authors of this progress report.

Haruo SATO Chair of the task group

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IASPEI Task Group on Scattering and Heterogeneity Progress reports on wave propagation and scattering since IUGG2003

Niren N. Biswas Geophysical Institute, University of Alaska Fairbanks 903 Koyukuk Drive, PO Box 757320 Fairbanks, AK 99775-7320, USA Email: [email protected] April 17, 2006

1. Researches 1.1 Attenuation characteristics of seismic energy in southcentral Alaska Coworkers: U. Dutta*, D.A. Adams** and A. Papageorgiou*** The determination of the attenuation characteristics of seismic energy in a propagating medium is an important problem of seismology. The total attenuation (Qt-1) can be expressed as the sum of scattering (Qs-1) and intrinsic (Qi-1) attenuations. In order to determine Qs-1 and Qi-1, Wu (1985) studied the spatial distribution of seismic waves for randomly heterogeneous elastic media based on radiative transfer theory. With this model, he computed Qi-1 and the seismic albedo, B0= Qs-1/ Qt-1 = ηs/(ηi + ηs), of the medium, where ηs (=kQs-1) and ηi(=kQi-1) are the scattering and intrinsic attenuation coefficients, respectively, and k is the wave-number. Wu’s (1985) method has been used to find the scattering and intrinsic attenuation coefficients in several seismogenic zones, for example, the Hindukush region (Wu and Aki, 1988), the northeastern United States and Canada (Toksöz et al., 1988), California (Mayeda et al., 1991), and southcentral Alaska (McSweeney et al., 1991). Hoshiba et al., (1991) pointed out that the infinite lapse time window assumption in Wu’s formulation may lead to erroneous estimations of Qs-1 and Qi-1 values, as a finite time window is necessary for computing the seismic energy from observed data. They calculated the spatiotemporal distribution of seismic energy (in contrast to the spatial distribution only yielded by Wu’s method) in a medium for an impulsive source with a uniform distribution of scatterers using a Monte Carlo simulation technique. Zeng et al. (1991) confirmed the results of Hoshiba et al (1991) with a time domain solution for a multiple isotropic scattering medium. Following the approach proposed by Hoshiba et al (1991), better known as the multiple lapse time window (MLTW) method, many workers have studied Qs-1 and Qi-1, for example, Fehler et al (1992) for the Kanto-Tokai area of Japan, Hoshiba (1993) for the entire Japan area, and Mayeda et al (1992) for Hawaii, Long Valley, and central California. However, Jin et al (1994) and Adams and Abercrombie (1998) used the MLTW method to estimate the scattering and intrinsic attenuation at different sites in California by an inversion technique using the integral solution of Zeng et al. (1991). A similar approach was also taken by Pujades et al. (1997) and Ugalde et al. (1998) to compute the intrinsic and scattering attenuation from seismic coda recorded by short-period seismograms in the southern Iberian Peninsula (Almeria basin) and in northeastern Venezuela, respectively. In the present study, we first obtained the direct S-wave (Qß-1) and coda wave (QC-1) attenuation values in southcentral Alaska using the generalized inversion and coda decay methods, respectively. Both the transverse and radial component data were used. The results showed that both Qß-1 and QC-1 are frequency dependent in the range of 0.6 to 10.0 Hz considered for this study. The frequency dependency for Qß-1 and QC-1 can be expressed in the form of a power law as ƒ 1.0-1.2 and ƒ 0.8-0.84, respectively. The difference in attenuation obtained between the transverse and the radial components for the direct S and coda waves is statistically insignificant. The closeness in frequency dependency between the two wave types (direct S and coda) suggests that the coda is primarily composed of S waves (Aki, 1980). However, the results show that for frequencies less than 3.0 Hz, Qß-1 is slightly higher than QC-1 but is less for frequencies greater than 3.0 Hz. This may be due to multiple scattering effects of the medium. The scattering (Qs-1) and intrinsic (Qi-1) attenuation of the medium were computed at three frequencies (1.0, 3.0 and 6.0 Hz) using MLTW method (Hoshiba et al., 1991). A comparison of different attenuation values in the frequency range of 1.0-6.0 Hz shows that QC-1 is nearly equal to the total attenuation Qt-1 (=Qs-1 + Qi-1) of the medium for ƒ ≥ 3.0 Hz, but at 1.0Hz, QC-1 values lie between Qt-1 and Qi-1. At 1.0Hz, the values of Qs-1 and Qi-1 are nearly the same, but with increasing frequency from 1.0 to 6.0 Hz, Qs-1 decreases faster (~ƒ-1.6) than Qi-1 (~ƒ-0.7), indicating a

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strong frequency dependence of Qs-1 in the medium. At higher frequencies (ƒ ≥ 3.0 Hz), the intrinsic attenuation becomes the dominant mechanism of S-wave attenuation in southcentral Alaska. Such dominance of the intrinsic attenuation in the study area appears to support the view of Frankel (1991) that the time decay of coda at higher frequencies (≥ 3.0 Hz) is controlled mainly by the intrinsic attenuation of the medium. The comparison of the results of QC-1 and Qt-1 with the student t-test yielded a significance value of 0.79; this shows that the difference between the coda attenuation and the total attenuation of the medium is statistically insignificant in the frequency range of 1.0-6.0Hz. It also is to be noted that the different attenuation values computed in this study were the average effect over the travel path weighted by the length of path segments through the different formations of the anchorage basin. The seismic albedo (B0) of the medium decreases with increasing frequency from 1.0 to 6.0 Hz, and the values seem to be consistent with those obtained for other seismogenic zones. The B0 values reported earlier by McSweeney et al. (1991) for south-central Alaska seem to be high and may be due to the application of Wu’s (1985) method used by McSweeney at al., as pointed out by Hoshiba et al. (1991) and Fehler et al. (1992). The low B0 ( 0.01), SISM underestimates the envelopes except for short lapse times. In contrast, EFM as well as DM appears to work well when multiple scattering is dominant. The agreement among the synthetic, EFM and DM envelopes is generally better for longer lapse times and/or higher c. Next, we compared the synthetic wave envelopes with the predictions of the radiative transfer theory (RTT), that is numerically obtained by Monte-Carlo simulations. We showed that the RTT envelopes are fairly consistent with the synthetic envelopes on the whole, especially when one properly considers the anisotropic scattering and uses the group velocities of waves (estimated by FAT) as the particle velocities in the Monte-Carlo simulations. Nevertheless, pulse broadening observed in low-frequency synthetic envelopes with long travel distances, that would be a result of waveform dispersion, is not reproduced by RTT. Although most of these results are theoretically expected in advance, the present study seems the first trial to experimentally confirm the validity and limits of the theories for media with discrete scatterers. Kawahara, J., and K. Yomogida (2003), Modeling of SH wave envelopes in media with many cavities, EOS Trans., Am. Geophys. Union, 84(46), Fall Meet., Suppl., Abstract S11E-0346. Kawahara, J., and K. Yomogida (2004), Modeling of SH wave envelopes in media with many cavities: wave simulations vs. radiative transfer theory, EOS Trans., Am. Geophys. Union, 85(47), Fall Meet., Suppl., Abstract S23B-0304. 1.2 Attenuation and dispersion of waves due to scattering in 2-D cracked media With Yuji Suzuki, Taro Okamoto and Kaoru Miyashita As mentioned above, FAT is a powerful tool to predict direct wave envelopes in media with discrete scatterers. Originally, FAT evaluates the expected values of attenuation and phase velocities of waves due to scattering in a stochastic sense. We numerically studied the attenuation and velocity dispersion due to scattering by 2-D stress-free cracks. Although such a problem has been preferably treated using boundary integral equation methods, we applied here a standard (basic) finite difference method to it and showed that the method can give sufficiently high accuracy. The key is to represent a crack with an array of grid points with zero shear traction and two-valued normal relative displacement. On the basis of this method, we let a

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plane wavelet incident on a cracked region and simulated the waves traveling through the region, in a similar manner as described in Research 1.1. Here we dealt with SH, SV and P wavelet incidence. We then evaluated the attenuation and velocity dispersion and compared them with the FAT predictions that we previously derived. We showed that the consistency between them is excellent for the crack densities up to 0.1 or more for S waves. The consistency is also confirmed for P waves, though the validity limit appears somewhat lower. In the case of SH waves, we also confirmed that FAT remains valid even for cracks with varying lengths. Suzuki, Y., J. Kawahara, T. Okamoto (2004), Simulations of P-SV waves scattered by 2-D cracks using the finite difference method, Jpn. Earth Planet. Sci. Joint Meet., Abstract S047-P005. Suzuki, Y., J. Kawahara, T. Okamoto, and K. Miyashita (2006), Simulations of SH wave scattering due to cracks by the 2-D finite difference method, Earth Planets Space, 58, 555-567. 2. Activities 2.1 Workshops Workshops on “Interpretation of physical properties of small scale heterogeneity in the lithosphere”, 20-21 Nov. 2003, 5-6 Jan. 2005, and 12-13 Jan. 2006, Earthquake Research Institute, the University of Tokyo, Tokyo, Japan. They were convened by J. Kawahara and financially supported by the "E.R.I. Cooperative Research Programs (2003-B-04)." Proceedings (in Japanese) are available at: http://wwweic.eri.u-tokyo.ac.jp/viewdoc/

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Michael Korn, Ulrich Wegler and Jens Przybilla

Institute of Geophysics and Geology, University of Leipzig Talstrasse 35, D-04103 Leipzig, Germany Email: [email protected]

1. Researches 1.1 Diffusive Waves in Layered Media Ulrich Wegler The problem of multiple scattering in a layer with transport mean free path l1 over a half-space with transport mean free path l2 was studied. The transport mean free path l2 of the half-space was also allowed to approach infinity reducing the problem to multiple scattering in a heterogeneous layer over a homogeneous half-space. To model this problem we used analytical solutionsof the diffusion equation as well as numerical Monte-Carlo simulations of the equation of radiative transfer. In the diffusion approach the major problem is to find suitable boundary conditions for the layer half-space interface. We found four different choices, which should be applied depending on the ratios of l1/l2 and l1/z1, where z1 is the layer thickness. For all of the four diffusion models analytical solutions were computed either in usual timespace domain or in frequency-wavenumber domain. The comparison to numerical solutions of the equation of radiative transfer demonstrated the broad validity ranges of the various diffusion approaches. We found that diffusion theory only fails, if the layer is thinner than its transport mean free path (z1 1Hz) seismic energy is indispensable for the understanding of earthquake source process and quantitative prediction of strong ground motion. Therefore, we developed a method to invert seismogram envelopes for the spatial distribution of high-frequency seismic energy radiation from earthquake faults (Nakahara et al. , 1998). Figure 1 is a schematic illustration of the method. There is a fault plane in a 3-D space. A solid star is the initial rupture point. Circles represent point-like isotropic scatterers randomly distributed. Being triggered by the rupture front with constant rupture velocity, seismic energy is radiated from a double couple source at the center of the k-th subfault, multiplly scattered during propagation, amplified beneath the i-th station (triangle) and reaches the station at the j-th time. The propagation process such as scattering and absorption is considered based on the radiative transfer theory (Sato et al., 1997). Seismic energy Wk and site amplification factor Si are estimated by inversion so as to minimize the residual between observed envelopes and synthesized ones. Rupture velocity and source duration time are estimated by the grid searche.

Nakahara, H., T. Nishimura, H. Sato, and M. Ohtake (1998), Seismogram envelope inversion for the spatial distribution of high-frequency energy radiation from the earthquake fault: Application to the 1994 far east off Sanriku earthquake, Japan, J. Geophys. Res., 103, 855-867. Sato, H., H. Nakahara, and M. Ohtake (1997), Synthesis of scattered energy density for the non-spherical radiation from a point shear dislocation source based on the radiative transfer theory, Phys. Earth Planet. Int., 104, 1-13.

Figure 1. Schematic illustration of the method of Nakahara et al. (1998).

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Observed characteristics of high-frequency seismic energy radiation from earthquakes We have so far applied the method of Nakahara et al. (1998) to 9 large earthquakes (listed in Table 1) with moment magnitude (Mw) of 5.9-8.3, eight of which occurred in Japan, and one of which in Taiwan. Three of them are the plate-boundary type, five are the shallow inland type, and one is the intra-slab type. Having compiled the results, the following statistical characteristics of the high-frequency seismic energy radiation can be extracted: (1) source duration time for subfaults, (2) scaling of radiated seismic energy with Mw, (3) statistical distribution of radiated seismic energy, (4) spatial distribution of radiated seismic energy on earthquake faults: (1) The duration time of energy radiation for each subfault is linearly proportional to the length of the subfault. (2) Seismic energy in four frequency bands of 1-2, 2-4, 4-8, and 8-16Hz was investigated in terms of Mw. The logarithm of the seismic energy was found to scale with Mw with the coefficient of close to 1.0. (3) Placing the seismic energy from all the subfaults for each event in the descending order, we clarified that the seismic energy systematically decreases with the order. This tendency seems to be independent of the frequency bands. The two-parameter Weibull distribution explains this tendency very well. One parameter, so called the Weibull parameter, ranges between 0 and 2 for the nine events, and controls the decay of seismic energy with the order; the larger parameter shows slower decay, and vice versa. (4) Systematic relation has not yet been obtained between locations of high-frequency seismic energy radiation and those of low-frequency wave radiation (or asperity); complementary for some events, but complex for other events.

Nakahara,H., (2003), Envelope inversion analysis for the high-frequency seismic-wave energy radiation by using Green functions in a depth-dependent velocity structure –The 2000 Western Tottori, JAPAN, earthquake -, IUGG, Sapporo, Japan. Nakahara,H., (2004), High-frequency envelope inversion analysis of the 2003 Tokachi-Oki, JAPAN, earthquake (Mw8.0),AGU 2004 fall meeting, San Francisco, USA. Nakahara, H., (2005), High-frequency envelope inversion analysis of the 2003 Miyagi-Ken-Oki earthquake (Mj 7.0)，Chikyu Monthly, 27, 39-43, (in Japanese). Nakahara, H., (2005), High-frequency envelope inversion analysis of the 2004 Niigata-Ken Chuetsu, JAPAN, earthquake (Mw6.6), 2005Japan Earth and Planetary Science Joint Meeting, Chiba, JAPAN. Nakahara, H., T. Nishimura, H. Sato, and M. Ohtake (1998),Seismogram envelope inversion for the spatial distribution of high-frequency energy radiation from the earthquake fault: Application to the 1994 far east off Sanriku earthquake, Japan, J. Geophys. Res., 103, 855-867. Nakahara, H., T. Nishimura, H. Sato, M. Ohtake, S. Kinoshita, and H. Hamaguchi (2002),Broadband source process of the 1998 Iwate Prefecture, Japan, earthquake as revealed from inversion analyses of seismic waveforms and envelopes, Bull. Seismol. Soc. Am., 92, 1708-1720. Nakahara, H., H. Sato, M. Ohtake, and T. Nishimura (1999), Spatial distribution of high-frequency energy radiation on the fault of the 1995 Hyogo-Ken Nanbu, Japan, Earthquake (MW 6.9) on the basis of the seismogram envelope inversion, Bull. Seismol. Soc. Am., 89, 22-35. Nakahara, H., R. Watanabe, H. Sato, and M. Ohtake (2005), Spatial distribution of high-frequency seismic energy radiation on the fault plane of the 1999 Chi-Chi, Taiwan, earthquake (Mw 7.6) as revealed from an envelope inversion analysis, Submitted to PAGEOPH.

Table 1. List of earthquakes so far analyzed Event, Mw

Source Location

1994 Off-Sanriku, JAPAN Plate Boundary （Mw7.7） 1995 Kobe, JAPAN （Mw6.9） Inland 1998 Northern Iwate, JAPAN （Mw5.8）

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Inland

Focal HF Location Reference Mechanism and Type LF Location Thrust Complementary Nakahara et al. (1998) Right-lateral Strike slip Reverse

2

Complex

Kakehi et al. (1996) Nakahara et al. (1999) Complementary Nakahara et al. (2002)

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1999 Chi-Chi, Taiwan （Mw7.6）Inland

Thrust

Complementary Nakahara et al. (2005)

2000 Western Tottori, JAPAN Inland （Mw6.7） 2003 Off-Miyagi, JAPAN Intra Slab （Mw7.0) 2003 Off-Tokachi, JAPAN Plate (Mw 8.3) Boundary Largest Aftershock of the 2003 Plate Off-Tokachi, JAPAN (Mw 7.3) Boundary 2004 Niigata Chuetsu, JAPAN Inland (Mw 6.6)

Left-lateral Strike slip Reverse

Complementary Nakahara (2003)

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Complex

Nakahara (2005)

Thrust

Complex

Nakahara(2004）

Thrust

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Nakahara(2004）

Complex

Nakahara (2005)

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Osamu NISHIZAWA National Research Institute of Advanced Industrial Science and Technology [email protected]

March 15, 2006 1. Researches *.1 Phase fluctuation in random media revealed by laboratory experiments With Genshiro Kitagawa A study of phase fluctuation in random media by applying a multivariate AR-model analysis Effects of small-scale heterogeneities on seismic-waveform fluctuations were studied by model experiments in laboratory. Elastic waves propagating through a granite block were recorded at 180 observation points that were arranged as an equally-spaced circular array. A disk-shaped PZT source was attached on the other side surface of the circular array for realizing equivalent receiver positions with respect to both source radiation pattern and travel distances of waves. A laser Doppler vibrometer was used for observing waveforms. To investigate fluctuations of waves, waveform pairs were selected out from the 180 waveforms and cross spectra of time-windowed partial waveforms were calculated by applying the multivariate AR model. By comparing waveforms of two observation points, the cross-spectral amplitudes and phases are obtained with respect to the lapse time by moving time window, or to the spatial distance by changing the pairs of observation points. We obtain distributions of cross-spectral phase values for frequency and the lapse time of waveforms. The distributions indicate phase fluctuation of waves in random media with respect to frequency and lapse time. Heterogeneity of the rock sample is expressed as a one-dimensional exponential autocorrelation functions (ACF); ε exp (-| r | / a), where r is the distance, and a and ε are the correlation length (0.22 mm) and the strength of heterogeneity (8.5 %), respectively. The distributions are plotted against ka; the product of wavenumber and correlation length. For small ka, the distributions of phase are close to the Gaussian distributions with small variances, but the variances quickly become large above ka ~ 0.2 - 0.3. Then the distributions become uniform between -π and π. This suggests that the incoherent scattered waves become dominant above a critical ka value (or a critical frequency for a medium), and phase information in later portions of waveforms will be lost. This may be important for extracting reflection, refraction or converted waves that are assigned as signals from geologic discontinuities because those signals may be strongly distorted by scattered waves produced from the small-scale heterogeneities of earth's media.

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(b) Reflected wave (58μs) distance skip 7 0.25 MHz source

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Figure 1. Amplitude (left) and phase (right) distributions of reflected waves in granite sample for the waveform pairs with the distance 7 station interval. Phase fluctuations determined by cross spectra show abrupt increases at certain frequencies.

Nishizawa, O. (2004) Experimental Studies of Waveform Fluctuation in Random Heterogeneous Media Proceedings of teh 7th SEGJ International Symposium -Imaging Technology-, Sendai, November 2004, 225-231.

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Haruo SATO Geophysics, Science, Tohoku University, Sendai, 980-8578, Japan [email protected] March 12, 2006

1. Researches 1.1 Envelope modeling in random media With Mike Fehler, Michale Korn, Tatsuhiko Saito, Takuto Maeda, Osamu Nishizawa, Yo Fukushima Direct simulation of envelope in random media High-frequency seismograms are complex and significantly different from those computed for conventional layered structures; however, their envelopes are repeatable, frequency dependent, and vary regionally. Recognizing the complexity of seismograms and Earth inhomogeneity, we focus on understanding envelopes of band-pass filtered traces rather than on unfiltered waveforms. Stochastic approaches are superior to deterministic wave-theoretical approaches for modeling wave envelopes in random media. Therefore, it is important to establish a way directly to simulate wave envelopes in random media. Sato and Fehler [2006] summarized a way to simulate scalar wave envelopes in 2-D random media by using the Markov approximation, radiative transfer approach and their hybrid use. A comparison with finite difference simulation confirmed the validity of the Markov approximation [Saito et al., 2003]. Korn and Sato [2005] first showed a way to simulate vector wave envelopes in 2-D random elastic media characterized by Gaussian ACF for the case that wavelength is shorter than the characteristic scale. Sato [2006] succeeded in deriving vector wave envelopes for plane wave propagation through 3-D random elastic media. Experimental study By using ultrasonic waves, Fukushima et al. [2003] experimentally clarified envelope broadening and collapse of linear polarization of S-wave with grain size increasing in rock samples. Saito, T., H. Sato, M. Fehler, and M. Ohtake (2003), Simulating the Envelope of Scalar Waves in 2D Random Media Having Power-Law Spectra of Velocity Fluctuation, Bull. Seismol. Soc. Amer., 93, 240-252. Fehler, M. and H. Sato (2003), Coda, Pure Appli. Geophys., 161, 541-554. Fukushima, Y., O. Nishizawa, H. Sato, and M. Ohtake (2003), Laboratory Study on Scattering Characteristics of Shear Waves in Rock Samples, Bull. Seismol. Soc. Amer., 93, 253-263. Sato, H., M. Fehler and T. Saito (2004), Hybrid Synthesis of Scalar Wave Envelopes in 2-D Random Media Having Rich Short Wavelength Spectra, J. Geophys. Res. 109, B06303, doi:10.1029/2003JB002673. Korn, M. and H. Sato (2005), Synthesis of plane vector wave envelopes in two-dimensional random elastic media based on the Markov approximation and comparison with finite-difference simulations, Geophys. J. Int., 161, 839-848. Sato, H. and M. Fehler (2006), Synthesis of Seismogram Envelopes in Heterogeneous Media in "Wave Propagation in Heterogeneous Earth" (Eds. R. S. Wu and V. Maupin) in "Advances in Geophysics” (Series Ed, R. Dmowska), Elsevier, in press. Sato, H. (2006) Synthesis of vector-wave envelopes in 3-D random elastic media Synthesis of vector-wave envelopes in 3-D random elastic media characterized by a Gaussian autocorrelation function based on the Markov approximation I: Plane wave case, J. Geophys. Res. Solid Earth, in press.

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Figure 1. Direct simulation of MS envelopes of plane P-waves propagating to the z direction through random elastic media on the basis of the Markov approximation [Sato, 2006]

1.2 Quantification of earth medium inhomogeneity With Won Sang Lee, Takuto Maeda, Tsutomu Takahashi, Takeshi Nishimura Coda waves for a long lapse time range Coda waves are the most prominent evidence of scattering due to Earth heterogeneity. Analyzing S coda envelopes registered by IRIS network in the world, we find a bend before and after ScS arrivals. Applying the isotropic scattering model to S coda envelopes, we estimated scattering coefficients in the upper and the lower mantle at 4s and 10s periods. The scattering coefficients estimated are smaller than those in the lithosphere but large enough to excite coda waves [Lee et al., 2003, 2006a]. Analyzing S coda for a long lapse time range up to 4000s, we find S-coda envelope decays according to a power of lapse time in general [Lee et al., 2006b]. Analyzing vertical component seismograms in long periods from 100 to 200s for a very long lapse time range up to 20 hours, we find a dominance of the fundamental mode before 30,000s; however, higher modes of spheroidal oscillation dominate after that. It was clarified from the f-k analysis and coda Q analysis [Maeda et al., 2003; 2006]. Envelope broadening and amplitude attenuation Scattering simultaneously causes envelope broadening and peak decay with travel distance increasing. Applying the Markov approximation model to observed S-wave envelopes of microearthquakes in northern Honshu, Japan, Saito et al. [2005] succeeded in explaining both maximum amplitude decay and envelope broadening with travel distance increasing. Temporal change in medium inhomogeneity Applying cross spectral analysis to seismic records of repeated artificial explosions in northeastern Honshu, Japan, we find that the seismic velocity at the frequency range of 3–6 Hz decreased by about 1% associated with the occurrence of earthquake of M6.1 near around the focal region and a gradual recovery of velocity for five years since then [Nishimura et al., 2005]. Receiver function analysis Analyzing H-net data in western Japan, we clarified a boundary of the subducting Pacific plate beneath Shikoku [Shiomi et al., 2004]. Joint analysis of Hi-net data and temporal observation data clarified the spatial variation of Moho depth in relation with the Itoigawa-Shizuoka tectonic line in central Japan [Yoshimoto et al., 2004]. Maeda, T., H. Sato, and M. Ohtake (2003), Synthesis of Rayleigh-wave envelope on the spherical Earth: Analytic solution of the single isotropic-scattering model for a circular source radiation, Geophys. Res. Lett., 30, 1286-, doi:10.1029/2002GL016629. Lee, W. S., H. Sato, K. Lee (2003), Estimation of S-wave scattering coefficient in the mantle from envelope characteristics before and after the ScS arrival, Geophys. Res. Lett., 30, No.24, 2248, doi:

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10.1029/2003GL018413. Shiomi, K., H. Sato, K. Obara, and M. Ohtake (2004), Configuration of subducting Philippine Sea plate beneath southwest Japan revealed from receiver function analysis based on the multivariate autoregressive model, J. Geophys. Res., 109, B04308, doi:10.1029/2003JB002774. Yoshimoto, K., H. Fujisawa, T. Okada, N. Umino, A. Hasegawa, K. Obara, K. Shiomi, H. Tsukahara, S. Okamoto, T. Kawanaka, H. Sato, T. Nishimura, Haruo Sato, and M. Ohtake (2004), Moho and Philippine Sea plate structure beneath central Honshu Island, Japan, from teleseismic receiver functions, Earth Planets Space, 56, 1271-1277. Takahashi, T., H. Sato, M. Ohtake and K. Obara (2005), Scale-dependence of apparent stress for earthquakes along the subducting Pacific Plate in Northeastern Honshu, Japan, Bull. Seismol. Soc. Am., Vol. 95, No. 4, pp. 1334-1345, doi: 10.1785/0120040075. Saito, T., H. Sato, M. Ohtake, and K. Obara (2005), Unified Explanation of Envelope Broadening and Maximum-Amplitude Decay of High-Frequency Seismograms based on the Envelope Simulation using the Markov Approximation: Fore-arc side of the Volcanic Front in Northeastern Honshu, Japan, J. Geophys. Res, 110, B01304, doi: 10.1029/2004JB003225. Nishimura, T., S. Tanaka, T. Yamawaki1, H. Yamamoto, T. Sano, M. Sato, H. Nakahara, N. Uchida, S. Hori, and H. Sato (2005)Temporal changes in seismic velocity of the crust around Iwate volcano, Japan, as inferred from analyses of repeated active seismic experiment data from 1998 to 2003, Earth Planets Space, 57, 491-505. Lee, W. S., H. Sato and K. Lee (2006a), Scattering coefficients in the mantle revealed from the seismogram envelope analysis based on the multiple isotropic scattering model, Earth Planet. Sci. Lett., 241, 888–900. Lee, W. S., and H. Sato (2006b) Power-law decay characteristic of coda envelopes revealed from the analysis of regional earthquakes", Geophys. Res. Lett, in press. Maeda, T., H. Sato and M. Ohtake (2006), Constituents of vertical-component coda waves at long periods, Pure and Appl. Geophys., in press.

2. Activities 2.1 Workshops Satellite Workshop of IUGG 2003 on "Seismic Waves in the Heterogeneous Earth: More Applications to Seismology and Exploration Geophysics", 7/10/2003, convened by H. Sato and H. Niitsuma, Tohoku Univ., Sendai, Japan. Workshop on “Probing Earth Media Having Small-Scale Heterogeneities”, 11/22/2004, convened by H. Sato, O. Nishizawa and H. Asanuma, Tohoku Univ., Sendai, Japan Session on “Scattering and Attenuation” at IASPEI, 10/03/2005, Santiago, Chile, chaired by M. Korn. There have been annual meetings on seismic wave scattering at ERI in Tokyo. Proceedings (in Japanese) are available: http://wwweic.eri.u-tokyo.ac.jp/viewdoc/scat2006.pdf, http://wwweic.eri.u-tokyo.ac.jp/viewdoc/scat0501/2003-B-04-2004.pdf, http://wwweic.eri.u-tokyo.ac.jp/viewdoc/scat0311/scat0311.pdf. Forthcoming meetings are Session of “Seismic wave propagation” convened by Nishizawa, Sato et al. and a session of “Memorial session for late Kei Aki” convened by Yomogida, Sato et al. at the Japan Geoscience Union Meeting 05/14-18/2006 at Makuhari, Japan, where seismic wave scattering related studies will be presented. 2.2 Web site “Scattering and Heterogeneity in the Earth” For exchanging scientific information related to seismic wave scattering in the heterogeneous earth medium, we have operated a special web site since 2001: http://www.scat.geophys.tohoku.ac.jp/. News about workshops and recent publications are uploaded and key references are updated. There is a plan to add a tutorial course and a discussion board.

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Tatsuhiko Saito AIST, Tsukuba, Japan [email protected]

March 24, 2006 1. Researches The idea of random media where velocity value fluctuates randomly in space is widely accepted for modeling velocity inhomogeneity in lithosphere. In the random media, wave propagation is represented as a stochastic wave equation. Statistical properties of the wavefield are investigated with mathematical and numerical methods. Also, we estimated the random inhomogeneity beneath Japan. 1.1 Wave envelope in random media with Haruo Sato, Michael Fehler, Obara Kazushige, Masakazu Ohtake By using the small-angle scattering approximation, wave envelopes in random media can be directly simulated from the power spectral density function (PSDF) of the random media [e.g. Sato 1989; Saito et al. 2002]. The theoretical envelope is referred to as Markov envelope, hereinafter. By analyzing the observed envelopes based on the Markov envelope, we estimated the PSDF of the velocity fluctuation beneath northeastern Honshu, Japan [Saito et al. 2005]. The estimated PSDF can successfully explain both the duration and the maximum amplitude of the envelope against the travel distance. Since the above method assumed small-angle scattering approximation, we focused on only the early part of observed envelopes. On the other hand, the later part of envelope or coda envelope is composed of the waves with the large-angle scattering. The radiative transfer theory can simulates the large-angle scattering well. We propose new methods which combine the Markov envelope and the envelope of the radiative transfer theory [Saito et al. 2003; Sato et al. 2004] to include both the small-angle and large-angle scattering. Many conventional studies assumed that random inhomogeneity was isotropic in space. However, anisotropic inhomogeneity where the horizontal correlation distance is larger than the vertical correlation distance is more realistic for the inhomogeneity in lithosphere. Therefore, Saito [2006a] formulated the envelope synthesis in 2-D anisotropic random media. The theoretical envelope shows directivity; envelopes around the onset decrease in the maximum amplitude more rapidly and increase in the duration for the horizontal propagation than for the vertical propagation. There was an inconsistency between the early part and the later part of the observed envelopes [Saito et al. 2005]; the inhomogeneity estimated from the early part of the envelope could not explain enough coda excitation. The directivity of the envelope due to the anisotropic random media can be one of the keys to solve this inconsistency.

(a) Vertical (z - axis ) propagation 0.25

r = 50km

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75km

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100km 0.15

125km

Figure 1. Root-mean-square envelopes of waves for 2Hz Ricker wavelet source in 2-D Gaussian-type random media of V0 = 4km/s, ax = 5km, az = 2.5km, and ε = 0.05. (a) Propagation along z axis and (b) propagation along x axis. Each numeral shows the distance from the source. Bold-solid curves and bold-dashed curves show RMS envelopes based on the Markov approximation and those by FD simulations, respectively. Fine-dashed curves show ±1 standard deviation of FD envelopes.

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1.2 Velocity shift in random media When propagating through random media, waves prefer high velocity zones. As a result, waves effectively propagate faster than the spatial average velocity. The discrepancy between the effective average velocity and the spatial average velocity is referred to as velocity shift [e.g. Shapiro et al. 1996; Samuelides 1998]. Saito [2006b] formulated the velocity shift in 2-D anisotropic random media. The theory predicts that wave effectively propagates faster than the horizontal wave propagation than in the vertical propagation. Observations of P-wave velocity anisotropy have usually been interpreted in terms of preferred orientations of cracks and minerals in past studies. However this study indicates that wave scattering due to anisotropic random media can provide an alternative explanation for those observations.

Figure 2. Velocity shift versus the angle of incidence in anisotropic in 2D anisotropic random media characterized by an Gaussian auto-correlation function with ε = 0.05, ax = 80m, az = 40m, and Vo = 2700m/s at a travel distance of L = 520m for the frequency 40, 80 and 120Hz. Solid curves are calculated with the Rytov method and shaded areas are estimated from wave propagation simulations.

Saito, T., H. Sato, M. Fehler, and M. Ohtake (2003), Simulating the envelope of scalar waves in 2D random media having power-law spectra of velocity fluctuation, Bull. Seismol. Soc. Amer., 93, 240-252. Sato, H., M. Fehler and T. Saito (2004), Hybrid Synthesis of Scalar Wave Envelopes in 2-D Random Media Having Rich Short Wavelength Spectra, J. Geophys. Res. 109, B06303, doi:10.1029/2003JB002673. Saito, T., H. Sato, M. Ohtake, and K. Obara (2005), Unified Explanation of Envelope Broadening and Maximum-Amplitude Decay of High-Frequency Seismograms based on the Envelope Simulation using the Markov Approximation: Fore-arc side of the Volcanic Front in Northeastern Honshu, Japan, J. Geophys. Res, 110, B01304, doi: 10.1029/2004JB003225. Saito, T. (2006a), Synthesis of scalar-wave envelopes in two-dimensional weakly anisotropic random media by using the Markov approximation, Geophys. J. Int., in press. Saito, T. (2006b), Velocity shift in 2-D anisotropic random media using the Rytov method, Geophys. J. Int., in press.

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Francisco J. Sánchez-Sesma Instituto de Ingeniería UNAM Circuito Escolar s/n Cd. Universitaria, Coyoacán 04510 México D. F. MEXICO Tel (+52 55) 5623-3655 Fax (+52 55) 5616-1514 E-mail: [email protected] April 29, 2006

1. Summary of research

This Progress Report deals with the Green function retrieval from correlations in two canonical examples, namely the elastic 2D and 3D cases and the 2D case of a heterogeneous medium formed by an elastic cylindrical inclusion embedded in a full space. The first is a collaboration with Michel Campillo of the Joseph Fourier University of Grenoble, France, and consists in demonstrating analytically that for isotropic plane waves in an elastic medium the Fourier transform of the azimuthal average of the cross correlation of motion between two points within the medium is proportional to the imaginary part of the exact Green tensor function between these points, provided isotropical illumination of plane waves and that the energy ratio ES/EP is the one predicted by Equipartition in two- and three-dimensions, respectively. In 2D we have

[

]

ui (y, ω )u*j (x, ω ) = −8ES k − 2 Im Gij (x, y, ω ) with ES = ρω 2 S 2 / 2 While in 3D we obtain

[

ui (y , ω )u *j (x, ω ) = −4π ES k − 3 Im Gij (x, y;ω )

]

These results clearly show that Equipartition is a necessary condition to retrieve the exact Green function from correlations of the elastic field (Sánchez-Sesma and Campillo, 2006)

SV

P x

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y

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Fig. 2. Propagation of plane P, SV and SH waves in 3D

The second canonical problem studied is the retrieval of the 2D elastodynamic Green function in an infinite elastic medium containing a circular cylinder inclusion. This work has been completed in collaboration with 06/30/06

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Juan Alfonso Pérez-Ruiz and Francisco Luzón of the University of Almería, Spain, and Michel Campillo of Grenoble, France. As in the former article the elastic space is isotropically illuminated with plane waves. The complete analytical solution was developed for the anti-plane SH case and for the in-plane P-SV waves as well. It is assumed a uniform spectra for both P and SV waves and such that the energy ratio ES/EP = (α/β)2, which is the one predicted by Equipartition theory in two-dimensions. Then it is shown that the Fourier transform of azimuthal average of the cross-correlation of motion between two points within an elastic medium is proportional to the imaginary part of the exact Green tensor function between these points (Sánchez-Sesma et al., 2006). REFERENCES Sánchez-Sesma, F. J. and M. Campillo (2006), Retrieval of the Green function from cross-correlation: The canonical elastic problem, Bull. Seism. Soc. Am., 96, in press. Sánchez-Sesma, F. J., J. A. Pérez-Ruiz, M. Campillo and F. Luzón (2006). The elastodynamic 2D green function retrieval from cross-correlation: the canonical inclusion problem, Geophysical Research Letters, submitted.

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Tsutomu Takahashi IFREE/JAMSTEC, Showa-machi 3173-25, Kanazawa-ku, Yokohama, Kanagawa 236-0001 JAPAN ttaka @ jamstec.go.jp

April 13, 2006 1. Researches 1.1 Scale dependence of apparent stress of small- and moderate sized earthquakes With Haruo Sato, Masakazu Ohtake and Kazushige Obara Apparent stress of microearthquakes reflects the relative strength of high-frequency wave radiation on seismic source. In order to measure this quantity, adequate corrections for scattering, attenuation and site amplification factors are quite necessary especially in high frequency range (>1Hz). We estimated the apparent attenuation of S-wave and site amplification factors by the coda normalization method. By using these parameters estimated, we examined whether the apparent stress depends on seismic moment or not. Finally, we concluded that apparent stress for small and moderate sized earthquakes occurred around subducting Pacific plate show clear scale dependence. This result can not be attributed to the underestimation of attenuation factors or other artificial causes. Takahashi, T., H. Sato, M. Ohtake and K. Obara (2005), Scale-dependence of apparent stress for earthquakes along the subducting Pacific Plate in Northeastern Honshu, Japan, Bull. Seismol. Soc. Am., Vol. 95, No. 4, pp. 1334-1345, doi: 10.1785/0120040075. 1.2 Classification of seismic envelopes in northeastern Japan With Haruo Sato, Takeshi Nishimura and Kazushige Obara It has been recognized that seismic envelopes of microearthquakes observed in the back-arc side of the volcanic front in northeastern Honshu, Japan are more strongly broadened in higher-frequencies (>1Hz), even though those observed in the fore-arc side are weakly broadened and independent on frequency [Obara and Sato (1995)]. In this study, examining seismic envelopes recorded by a dense seismic network (Hi-net, NIED) in this area, we revealed the clear path dependence of envelope broadening and its relation to the Quaternary volcanoes. We mainly analyzed the peak delay time of S-wave envelope which is defined as the time lag from S-wave onset to S-wave peak arrival, because this quantity is insensitive to intrinsic absorption. The observed path-dependence of peak delay time can be summarized as follows: Peak delay times tend to be small for the waves propagating in the fore-arc side of the volcanic front and also in the non-volcanic area in the back-arc side. Large peak delay times in the back-arc side are observed only for the ray-paths propagating beneath Quaternary volcanoes. 1.3 Envelope modeling in non-uniform random media With Haruo Sato, Takeshi Nishimura To consider the path-dependence of envelope broadening mentioned above, we have to know the characteristics of envelope broadening in spatially non-uniform random media. The Markov approximation of parabolic wave equation is a very useful approach for the direct synthesis of wave envelopes. However, theoretical studies of the Markov approximation are established only for spatially uniform random media. We employ the stochastic ray-path method of the Markov approximation [Williamson (1972)] to derive envelopes in non-uniform media because it is easy to consider the variation of random inhomogeneities along seismic ray-path. Considering spherically outgoing waves from a point source and von Karman type power spectram density function of velocity fractional fluctuation, we examined the characteristics of envelope broadening in various non-uniform random media. Finally we successfully find a simple relation between the frequnecy dependence of peak delay and the inhomogneity power spectrum.

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1.4 Inversion of peak-delay time for quantification of medium inhomogeneities With Haruo Sato, Takeshi Nishimura, Kazushige Obara By using simple method calculating the peak delay time in spatially non-uniform random media (see 1.3), we inverted the observed peak delay time to estimate the power spectral density function of random velocity fluctuation in northeastern Japan. Since the obtained simple method is in non-linear form, we employed the genetic algorithm in our inversion. Assuming von Karman type random media, we successfully estimate a spatial distribution of inhomogeneity power spectral density in northeastern Japan. We find the following results; regions beneath Quaternary volcanoes are rich in short wavelength power spectra. Western part of Hidaka Mountains in 20-40km depth show strong velocity inhomogeneity.

2. Activities 2.1 Web site “Scattering and Heterogeneity in the Earth” For exchanging scientific information related to seismic wave scattering in the heterogeneous Earth medium, we have operated a special web site of our task group since 2001: http://www.scat.geophys.tohoku.ac.jp/. News about workshops, recent publications and key references are uploaded. Recently a discussion board was opened. We have a plan to open a tutorial course.

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Arantza UGALDE Ebro Observatory, CSIC – URL, Horta Alta, 38, 43520-Roquestes (Tarragona), Spain [email protected] April 3, 2006 1

Researches

1.1 Estimation of coda-wave attenuation With Carlos A. Vargas, Lluís G. Pujades, Josep A. Canas, Jayant N. Tripathi, Mitsuyuki Hoshiba and Bal Rastogi Spatial variation of coda-wave attenuation in north-western Colombia 1786 vertical-component, short period observations of microearthquake codas from regional earthquakes recorded by 17 stations belonging to the National Seismological Network of Colombia were used to estimate seismic wave attenuation in Colombia. Local magnitudes range from 2.9 to 6.0 and only events occurring at hypocentral distances up to 255 km were considered for the analysis. The frequencies of interest lay between 1 and 19 Hz and the analysis was performed for each seismic station separately. Codawave attenuation (Qc-1) was estimated by means of a single scattering method whereas the separation of intrinsic absorption (Qi-1) and scattering attenuation (Qs-1) from total attenuation (Qt-1) was performed using a multiple lapse time window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers. A regionalization of the estimated Q0 (Qc at 1 Hz) values was performed and a contour map of seismic coda attenuation in Colombia is presented, where four zones with significant variations of attenuation related to different geological and tectonic characteristics can be observed. The highest attenuation is linked to the central and western regions (Q0 around 50 and 56) whereas a lower attenuation (Q0 around 69 and 67) is assigned to the northern and eastern regions. Results show that the Q-1 values are frequency dependent in the considered frequency range, and are approximated by a least-square −η fit to the power law Q −1 ( f ) = Q0 −1 ( f / f 0 ) . The exponents of the frequency dependence law ranged from

η=0.65 to 1.01 for Qc-1, η =0.62 to 1.78 for Qi-1, η =0.28 to 1.49 for Qs-1, and η =0.53 to 1.67 for Qt-1. On

the other hand, intrinsic absorption is found to dominate over scattering in the attenuation process for most of the stations and frequency bands analyzed. Some discrepancies have been observed between the theoretical model and the observations for some frequency bands which indicate that it would be necessary to consider models for depth dependent velocity structure and/or non-isotropic scattering pattern.

OCA 8

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Figure 1. Q0 (Qc at 1 Hz) estimates for the Colombian territory from a regionalization method based on the midpoints of the epicentre-station paths. Active volcanoes are represented by triangles and the stations by stars

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Regional estimation of Q from seismic coda observations by the Gauribidanur seismic array (southern India) Attenuation properties of the lithosphere in southern India are estimated from 1219 verticalcomponent, short period observations of microearthquake codas recorded by the Gauribidanur seismic array. The magnitudes of the earthquakes range from 0.3 to 3.7 and have focal depths less than 10 km. Coda-wave attenuation (Qc-1) is estimated by means of a single isotropic scattering method and a multiple lapse time window analysis based on the hypothesis of multiple isotropic scattering and uniform distribution of scatterers is used to estimate the contribution of intrinsic absorption (Qi-1) and scattering (Qs1 ) to total attenuation (Qt-1). All the attenuation parameters are estimated, as a function of frequency for hypocentral distances up to 255 km. Results show a frequency dependent relation of the Qc-1 values in the range 1 to 10 Hz that fit the power law Q-1(f)=Q0-1 (f/f0)η. A Q0-1 value of 0.014 and a decrease of f -1.2 have been found using data from the whole region. On the other hand, scattering attenuation is found to be greater than intrinsic absorption for all the frequency bands. A high value of the seismic albedo (which ranges from 0.68 to 1) is found which indicates that scattering is the dominant effect in the study region. Nevertheless, the attenuation parameters estimated are much lower than the obtained for other regions in the world. On the other hand, the observed energy at 0 to 15 seconds from the S wave arrival time bends significantly downward with decreasing distance. In order to clarify this phenomenon, there is a need to take into account the vertical varying velocity structure in the theoretical model. Intrinsic and scattering attenuation in western India from aftershocks of the 26 January, 2001, Kachchh earthquake 176 vertical-component, short period coda observations from aftershocks of the Mw 7.7, 26 January, 2001 Kachchh earthquake are used to estimate seismic wave attenuation in western India using uniform and depth dependent structure models. The magnitudes of the earthquakes are less than 4.5, with depths less than 46 km and hypocentral distances up to 110 km. The frequencies of interest lay between 1 and 30 Hz. Two seismic wave attenuation factors: intrinsic absorption (Qi-1) and scattering attenuation (Qs-1) are estimated using the Multiple Lapse Time Window method which compares time integrated seismic wave energies with synthetic coda wave envelopes for a multiple isotropic scattering model. We first assume spatial uniformity of Qi-1, Qs-1 and S-wave velocity (β), and then, we consider a synthetic model based on a vertical varying scattering coefficient (g), intrinsic absorption strength (h), density of the media (ρ) and β structure, which is computed using a Monte Carlo simulation method based on the hypothesis of multiple scattering for media consisting of several layers. Results show that, under the assumption of spatial uniformity, scattering attenuation is greater than intrinsic absorption only for the lowest frequency band (1 Hz to 2 Hz), whereas intrinsic absorption is predominant in the attenuation process for higher frequencies (2 Hz to 30 Hz). The values of Q obtained range from Qt=118, Qi=246 and Qs=227 to Qt≈4000, Qi≈4600 and Qs≈33300 at 1.5 and 28 Hz centre frequencies, respectively. Results also show that Qi-1, Qs-1 and Qt-1 decrease proportional to f -ν, however, two rates of decay are clearly observed for the low (1 Hz to 6 Hz) and high (6 Hz to 30 Hz) frequency ranges. Values of ν=2.07±0.05 and 0.44±0.09 for total attenuation, ν=1.52±0.21 and 0.48±0.09 for intrinsic absorption and ν=3.63±0.07 and 0.06±0.08 for scattering attenuation are obtained for the low and high frequency ranges, respectively. When considering a depth dependent model assuming 2 layers, results have a lower resolution, and they indicate that scattering attenuation is comparable to or smaller than the intrinsic absorption for the crust whereas the intrinsic absorption is the dominant effect in the mantle. Also, for a crustal layer with bottom at 42 km, intrinsic absorption and scattering estimates are lower and greater than those of the mantle, respectively. 0.001 0 km

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Vargas, C. A., A. Ugalde, L. G. Pujades, and J. A. Canas (2004), Spatial variation of coda-wave attenuation in north-western Colombia, Geophys. J. Int., 158, 609-624. Tripathi, J. N. and A. Ugalde (2004), Regional estimation of Q from seismic coda observations by the Gauribidanur seismic array (southern India), Phys. Earth Planet. Int., 145, 115-126. Ugalde, A., Tripathi, J. N., Hoshiba. M., and Rastogi, B. (2006a), Intrinsic and scattering attenuation in western India from aftershocks of the 26 January, 2001, Kachchh earthquake, Tectonophysics, in press.

1.2 Imaging scatterers in the earth’s crust With Eduard Carcolé, Carlos A. Vargas, and Jayant N. Tripathi Spatial distribution of scatterers in the crust by inversion analysis of coda envelopes: a case study of Gauribidanur seismic array (southern India) The three-dimensional spatial distribution of relative scattering coefficients in southern India was estimated by means of an inversion technique applied to coda wave envelopes. The inversion analysis was performed for the first time in this kind of seismological research by means of the Simultaneous Iterative Reconstruction Technique (SIRT) and Filtered Back-Projection method (FBP). Whereas the first one allows to obtain more exact solutions, the second one is a much faster non-iterative algorithm that has proved to provide very accurate reconstructions. Data used consisted of selected 636 vertical-component, short period recordings of microearthquake codas from shallow earthquakes with magnitudes ranging from 0.7 to 3.7 and epicentral distances up to 120 km recorded by the Gauribidanur seismic array (GBA). Results are almost independent of the inversion method used and they are frequency dependent. They show a remarkably uniform distribution of the scattering strength in the crust around GBA. However, a shallow (0-24 km) strong scattering structure, which is only visible at low frequencies, seems to coincide with the Closepet granitic batholith which is the boundary between the eastern and western parts of the Dharwar craton. 0

-20

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1-2 Hz 76.8

77

77.2

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Figure 3. Vertical cross section of relative scattering coefficients at the parallel 13.6º, which corresponds to the latitude of the array cross-point

α−1 -20

1 to 2 0.75 to 1 0.5 to 0.75 0.25 to 0.5 -0.25 to 0.25 -0.75 to -0.25

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Three-dimensional spatial distribution of scatterers in Galeras volcano, Colombia A three-dimensional spatial distribution of relative scattering coefficients is estimated for the Galeras volcano, Colombia, by an inversion of the coda wave envelopes from 1564 high quality seismic recordings at 31 stations of the Galeras seismograph network. The inversion reveals a highly non-uniform distribution of relative scattering coefficients in the region for the two analyzed frequency bands (4-8 and 8-12 Hz). Strong scatterers show frequency dependence, which is interpreted in terms of the scale of the heterogeneities producing scattering. Two zones of strong scattering are detected: the shallower one is

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located at a depth from 4 km to 8 km under the summit whereas the deeper one is imaged at a depth of ~37 km from the Earth’s surface. Both zones may be associated with the magmatic plumbing system beneath Galeras volcano. The second strong scattering zone may be related to a deeper magma reservoir that feeds the system.

Figure 4. Vertical cross section showing the results of the inversion analysis for a synthetic test consisting of two spherical structures buried at depths of -2 km and -33 km.

Figure 3. Vertical cross section of the study region along the two planes defined by the summit coordinates, which indicated by the solid triangle (latitude 1.23o and longitude -77.36o). The color scale indicates the perturbation of the scattering coefficientα-1 for the 4-8 Hz (A, B) and 8-12 Hz (C, D) frequency bands

Ugalde, A., Carcolé, E., and Tripathi, J. N. (2006b), Spatial distribution of scatterers in the crust by inversion analysis of coda envelopes: a case study of Gauribidanur seismic array (southern India), Geophys., J. Int., in press. Carcolé, E., Ugalde, A., and Vargas, C. A. (2006c), Three-dimensional spatial distribution of scatterers in Galeras volcano, Colombia, Geophys. Res. Lett., in press.

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Activities

International Workshop “CHALLENGES FOR GEOMAGNETISM, AERONOMY AND SEISMOLOGY IN THE XXI CENTURY”, on the occassion of the centennial of Ebro Observatory, Tortosa (Spain), September, 2004, Chaired by A. Ugalde. Information and contributions are still available at http://www.obsebre.es/php/workshop/

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Kazuo YOSHIMOTO International Graduate School of Arts and Sciences, Yokohama City University, 22-2 Seto, Kanazawa, Yokohama 236-0027, Japan [email protected]

March 17, 2006 1. Researches Spatial distribution of S-coda-wave energy and seismic attenuation structure The S-coda waves of local earthquakes had been considered as the multiple scattered waves that distribute uniformly in space at large lapse times. However, several recent studies have shown that the spatial distribution of S-coda-wave energy is not uniform in tectonically active regions. We had reported an apparent spatial variation of S-coda-wave energy in northeastern Honshu, Japan [Yoshimoto et al., 2006]. The S-coda-wave energy is uniformly distributed in the forearc-side, whereas a monotonic exponential decrease with horizontal distance from the volcanic front is found in the backarc-side. The decay rate shows clear frequency dependence and increases with frequency, suggesting strong attenuation of high-frequency waves in the backarc-side. By expanding regions, we are investigating the spatial distribution of S-coda-wave energy in Japan from the analysis of seismograms recorded by the Hi-net that consists of about 700 borehole seismometers. This study shows that the S-coda-wave energy does not distribute uniformly in space in Japan. The magnitude of S-coda-wave energy is systematically small in the area where the Quaternary volcanoes exist (e.g., central and northern Hokkaido, western Tohoku, Hokuriku, in and around the Izu peninsula, southern Kyushu). The magnitude of regional variation reaches up to 40 dB in the high-frequency range (16-32 Hz). The spatial correlation between the Quaternary volcanoes and the high heat flux suggests that the thermal structure (or intrinsic attenuation structure) of the crust and the uppermost mantle characterizes the regional variation of S-coda-wave energy in Japan.

45ºN -2

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0

1

2

Log (Coda-energy Factor)

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35ºN

130ºE

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145ºE

Figure 1. Map showing the regional variation of an index (coda-energy factor) that characterizes the magnitude of S-coda-wave energy in 16-32 Hz. The solid triangles show locations of the Quaternary volcanoes.

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Short-wavelength heterogeneity of the uppermost mantle In Japan, a long-duration coda wave after the PmP arrival is often observed on the record sections of refraction surveys. To explain these observations, we examined three possible structure models: (1) undulated Moho boundary model; (2) laterally extensive layering model; and (3) random heterogeneous uppermost mantle model [Iidaka et al., 2006]. From a comparative study between observations and simulations, it is concluded that the random heterogeneous uppermost mantel model is most adequate for explaining the amplitude and duration of the coda waves observed. The scattering coefficient of the uppermost mantle is slightly larger than that of lower crust, suggesting the active magmatism in island-arc region.

Receiver function analysis Analyzing H-net data obtained in central Honshu, Japan, we investigated the depth variation of the Moho and the upper boundary of the subducting Philippine Sea plate [Yoshimoto et al., 2004].

Journal articles Iidaka, T., T. Iwasaki, and K. Yoshimoto (2006), Nontransparent uppermost mantle in the island-arc region of Japan, Tectonophysics, in press. Yoshimoto, K., H. Fujisawa, T. Okada, N. Umino, A. Hasegawa, K. Obara, K. Shiomi, H. Tsukahara, S. Okamoto, T. Kawanaka, H. Sato, T. Nishimura, H. Sato, and M. Ohtake (2004), Moho and Philippine Sea plate structure beneath central Honshu Island, Japan, from teleseismic receiver functions, Earth Planets Space, 56, 1271-1277. Yoshimoto, K., U. Wegler, and M. Korn (2006), A volcanic front as a boundary of seismic attenuation structures in northeastern Honshu, Japan, Bull. Seismol. Soc. Amer., in press.

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