HR: 08:00h
AN: S41D-01    [Abstracts]
TI: Offshore Double-Planed Shallow Seismicity of the NE Japan Forearc Region Revealed by Hypocenter Determination Using the sP Depth Phase
AU: * Gamage, S S
EM: sng@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
AU: Umino, N
EM: umino@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai, 980-8578 Japan
AU: Kirby, S
EM: skirby@usgs.gov
AF: U.S. Geological Survey, Menlo Park, California, 94025-3591 United States
AB: Precise locations of earthquakes in subduction zones are required to determine the geometry and structure of the plate interface and slab structure. High-resolution relocation for offshore earthquakes is especially difficult using conventional land-based seismic networks. In this study, we use a special depth phase often detected in shallow seismic events in the northeastern Japan forearc region and use it to redetermine focal depths. This distinct later phase is often observed in vertical component seismograms between direct P and S wave arrivals at epicentral distances of about 150 km or more for shallow earthquakes. This sP phase is an upgoing S wave from the focus which is then reflected and converted to P wave at the Earth_fs surface, diving into the Earth again, and finally arriving observation stations (Umino et al., 1995). The arrival time difference between this phase and the first P wave is very sensitive to the focal depth of the event, and hence we used this phase for the depth determination. We also determined focal mechanisms of the relocated shallow events by using P wave first motion data. In the relocation procedure, we use Japanese University Network data, Hi-net and JMA data for the period of 2000 to 2004. The distribution of relocated earthquakes clearly shows the configuration of the plate boundary at shallow depths beneath the Pacific Ocean. The Pacific plate in this region initially subducts at an extremely low dip angle and then gradually becomes a much steeper dip angle at a depth of approximately 30 km. We were also able to detect some events whose focal depths are significantly deeper than those of the interplate events. These events are apparently the near-trench counterpart of the lower plane of the double seismic zone beneath the land. Thus a double seismic zone is also formed near the Trench. For both northern and southern part of the NE Japan forearc region, most of the relocated upper plane events have low angle thrust fault type mechanisms, especially near the coastline. Although P wave initial motion data are not ideally distributed on the focal sphere, preliminary work indicates that upper-plane events occurring near the trench of the northern part of the subduction zone are normal fault type. However, due to the lack of mechanism data, we are unable to recognize such a pattern in the southern part of the study area. We also show that seismicity is high and with normal faulting mechanisms near the rupture area of 1933 Sanriku tsunami earthquake (M8.3). Preliminary results indicate that most of the upper plane events under the offshore of NE Japan are thrust faulting type and near the trench are normal faulting type. Bending and unbending of the Pacific plate appears to be the most natural explanation of the distribution and focal mechanism of these offshore events.
UR: http://aob-new.aob.geophys.tohoku.ac.jp/$\sim$sng/
DE: 7230 Seismicity and seismotectonics
DE: 7299 General or miscellaneous
DE: 8123 Dynamics, seismotectonics
DE: 7200 SEISMOLOGY
SC: Seismology [S]
MN: 2004 AGU Fall Meeting