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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2142 Three-dimensional seismic velocity structure in the Off-Miyagi and Off- Fukushima forearc Region Mr. Yojiro Yamamoto RCPEV, Gradiate School of Science Tohoku University IASPEI Ryota Hino, Kensuke Suzuki, Tomoaki Yamada, Masanao Shinohara, Toshihiko Kanazawa, Masayuki Tanaka, Yoshiyuki Kaneda, Kenji Uehira The Japan Trench is a plate convergent zone where the Pacific Plate is subducting below the NE Japan arc. Interplate coupling along the plate interface is estimated to be strong by a backslip modeling of the land GPS observation in the middle to southern part of the arc (Suwa et al., 2006). However, the middle part (the off-Miyagi region) and the southern part (the off-Fukushima region) show different characteristics of the interplate seismic activity. In the off Miyagi region, the large earthquakes (M > 7) with thrust mechanisms have occurred at an interval of about 40 years, and an interplate earthquake of M 7.2 occurred in this region on 16 August 2005. In the off-Fukushima region, few large interplate earthquakes have occurred while the background microseismicity is very high. In 2005, we deployed 51 OBSs in the off-Miyagi and off-Fukushima regions to observe the interplate seismicity, including the 2005 off-Miyagi earthquake (M7.2) and its aftershocks. Using the OBSs data with those of the onshore seismic stations, we performed a 3D seismic tomography in order to clarify whether there are some differences in the seismic velocity structures between the off-Miyagi and the off-Fukushima regions, corresponding to the differences in the seismic activity. In our results, the subducting oceanic crust and the mantle wedge of the overriding plate were imaged as the landward dipping low velocity (Vp: ~7.0 km/s, Vs: ~4.0km/s) layer and the high velocity (Vp: ~8.0 km/s, Vs: ~4.5km/s) layer above it, respectively. Most of the earthquakes were relocated along the plate boundary, including the mainshock and the aftershocks of 2005 earthquake. Comparing spatial extents of the rupture areas of the 1978 and 2005 earthquakes and the velocity variation in the mantle wedge, we found that the location of high Vp anomaly corresponds to the rupture areas of the large interplate earthquakes. This suggests that the heterogeneity of the upper plate delimits the extent of the rupture area of large interplate earthquakes. The Vp in the mantle wedge of the off-Fukushima region is slower than that in the off- Miyagi region. Previous airgun-OBS experiment studies indicated that there is a low Vp (~7.5 km/s) area at the toe of the mantle wedge in the off-Fukushima (Miura et al., 2003), while the Vp of the mantle wedge in the off-Miyagi is high (~8.0 km/s) (Ito et al., 2005, Miura et al., 2005). The Vp variation in the mantle wedge imaged by the present seismic tomography is consistent with their results. Keywords: subduction zone, velocity structure, mantle wedge
IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy (S) - IASPEI - International Association of Seismology and Physics of the Earth's Interior JSS011 Poster presentation 2143 Crustal and upper mantle structure beneath central South America deduced from ScS reverberation waveforms and receiver functions Dr. Keiko Kuge Department of Geophysics Kyoto University IASPEI Yusuke Okaue We investigated the crust and upper mantle structure beneath the central South America, approximately at 20 degree south, where the Nazca plate is subducting beneath the western coast and the Brazilian craton resides in the east. Our approach is to model ScS reverberation waveforms and receiver functions at GSN stations. ScS reverberation waveforms are sensitive to the path-averaged S-wave velocities, attenuation, and depths and reflection coefficients of discontinuities in the crust and mantle. Our procedure to determine these parameters follows that of Kato et al. (2001) utilizing ray-based theoretical waveform modeling and grid search optimization. Finiteness of source time duration is additionally taken into consideration in synthesizing waveforms of a great earthquake in this study. Depths of mantle discontinuities were separately estimated from arrival times of PS conversion phases on receiver functions. We also modeled receiver functions from early P coda to determine crustal structure. We found significant differences in the crust and uppermost mantle between the western coast and eastern continent. The Moho beneath the station NNA on the western coast is deeper than beneath the station BDFB in the eastern continent. While the Conrad and Moho beneath BDFB were estimated at depths of 22 and 40 km, respectively, discontinuities at depths of 25, 35, 60, and 70 km were detected beneath NNA. Some phases on the receiver functions imply an inclination of the discontinuities at 60-70 km depths, and the discontinuities can be the manifestations of the top of the subducting Nazca plate. A high value of Q (>400) was obtained for the upper mantle beneath NNA, but not beneath BDFB. This value is much higher than predicted from the expected mantle transition zone temperature, and an origin of this high Q might exist in the uppermost mantle. These observed differences between the east (NNA) and west (BDFB) could be attributed to the subducting Nazca plate. In the transition zone, the 410-km discontinuity is shallow beneath the western coast, which is consistently found in both studies. The mantle transition zone is thinner in the western coast (260 km) than in the eastern continent (270km), but it is thicker in either area than that beneath Japan. On the receiver functions in the western coast, we were able to identify a phase possibly from the 520-km discontinuity. Using SS reflection coefficients from the ScS reverberations and PS conversion coefficients from the receiver functions, we attempted to obtain constraints on density and Vs contrasts at mantle discontinuities. For the 410-km and 660-km discontinuities, the upper bounds of density contrast are 7- 9 % and 15-20 %, respectively, while the upper bounds of Vs contrast are 6-14 % and 7-20 % or more, respectively. Keywords: south america, crust, upper mantle
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