HR: 1340h
AN: S23B-0233    [Abstracts]
TI: Urgent aftershock observation of the intra-plate earthquakes under the Nankai trough axis using ocean bottom seismometers
AU: * Sakai, S
EM: coco@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yamada, T
EM: yamada@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hagiwara, H
EM: hagiwara@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Shinohara, M
EM: mshino@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kanazawa, T
EM: kanazawa@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Takanami, T
EM: ttaka@eos.hokudai.ac.jp
AF: Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo, 060-0810 Japan
AU: Hino, R
EM: hino@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: Shimizu, H
EM: shimizu@sevo.kyushu-u.ac.jp
AF: Institute of Seismology and Volcanology, Faculty of Science, Kyushu University, Shimabara, 855-0843 Japan
AU: Obana, K
EM: obanak@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kodaira, S
EM: kodaira@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AU: Kaneda, Y
EM: kaneday@jamstec.go.jp
AF: Japan Agency for Marine-Earth Science and Technology, 2-15 Natsushima-cho, Yokosuka, 237-0061 Japan
AB: The 2004 off the Kii Peninsula earthquake occurred on September 5, 2004. Knowing the precise aftershock distribution is important for understanding the mechanism of this earthquake. However, the hypocenter of the main shock was located more than 100 km offshore from the nearest station of the land observation network. In the three days after the main shock, we started ocean bottom seismometer (OBS) observation in order to determine the precise distribution of the aftershocks. We assumed a seismic velocity structure for the hypocenter calculation, based on the results of previous seismic refraction study. The station corrections were incorporated to locate the hypocenter precisely. The hypocenters located within an area covered by five OBSs show relatively small errors. It is found that the OBS-located hypocenters are located about 5.5 km east-southeast from those by JMA and the depth range of the aftershocks is about 5-25 km just beneath the Nankai trough axis. The aftershock hypocenters can be grouped into two clusters at different depths of about 10 km and about 20 km. It is inferred that the main shock also has a depth of 5-25 km. Since this extent of the main shock was larger than one of the oceanic crust of the Philippine Sea plate, the fault plane of the main shock extended at the upper most mantle of the Philippine Sea plate.Although we cannot assign the actual fault plane of the main shock form our observation results, it is clarified that intra-plate earthquakes occurred near the trench region. Our OBS result supports that the main shock was the earthquake not at the plate boundary but within the bending Philippine Sea plate near the trough axis.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
SC: Seismology [S]
MN: Fall Meeting 2005