Crust and Upper Mantle Structure From Seismic Refraction and Wide-Angle Reflection Surveys and its Relation to Seismic Activity in Southwest Japan

HR: 1340h
AN: T43A-1364 [Abstracts]
TI: Crust and Upper Mantle Structure From Seismic Refraction and Wide-Angle Reflection Surveys and its Relation to Seismic Activity in Southwest Japan
AU: * ITO, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokanosho, Uji, 611-0011 Japan
AU: Sato, H
EM: satow@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hirose, I
EM: issei@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokanosho, Uji, 611-0011 Japan
AU: Umeda, Y
EM: umeda@rcep.dpri.kyoto-u.ac.jpa
AF: Disaster Prevention Research Institute, Kyoto University, Gokanosho, Uji, 611-0011 Japan
AU: Hirata, N
EM: hirata@eri.u-rokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Ito, T
EM: tito@earth.s.chiba-u.ac.jp
AF: Faculty of Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba, 263-8522 Japan
AU: Abe, S
EM: sabe@jgi.co.jp
AF: JGI, 1-5-21 Ootshuka, Bunkyo-ku, Tokyo, 112-0012 Japan
AU: Kawanaka, T
EM: taku@jgi.co.jp
AF: JGI, 1-5-21 Ootshuka, Bunkyo-ku, Tokyo, 112-0012 Japan
AU: Ikawa, T
EM: ikawa@jgi.co.jp
AF: JGI, 1-5-21 Ootshuka, Bunkyo-ku, Tokyo, 112-0012 Japan
AB: Seismic surveys of the crust and the upper mantle structure have been conducted under the special project for earthquake disaster mitigation in urban areas (DAIDAITOKU). In 2004 deep seismic profilings were carried out in the Kinki district southwest Japan. In the area great earthquakes have been repeatedly occurred every 100 years on the Pacific Ocean side due to the subduction of the Philippine Sea plate (PSP). In addition, active faults are densely developed on shore of Japanese Islands, where large historical earthquakes with M7 or greater attacked large cities, such as Kobe, Kyoto and Osaka. The surveys aim at determining of source faults, propagation paths and site responses for big earthquakes. More specifically, purposes of the surveys are to determine the accurate positions and geometry of source faults, subducting plates and mega-thrust faults, crustal structure, seismogenic zone, sedimentary basins, and 3D velocity structures. These information leads to more realistic 3D estimation of strong motion for large earthquakes. In this study, a seismic refraction and wide-angle-reflection survey was carried out along the Shingu-Maiduru line, which crosses Honshu Island from the Pacific side to the Japan Sea side nearly coincident with the direction of the subduction direction of PSP. In the profile, 13 shots (100-700kg) of dynamite and 3 multivisweeps (a few hundreds of sweeps) with 4 vibroseis trucks were used as sources of refraction and wide-angle reflection surveys. More than 2000 seismometer stations were set at intervals of 50-100m all along the line. Clear first arrivals and reflections from PSP and from reflectors in the crust were obtained at almost all the stations. In particular, waves from dynamite were well recorded at all stations. As for the stacked data of multisweeps, waves can be recorded in the offset range of 30-40km from the source. The multivibration method is particularly useful for the surveys in the urban areas, where dynamite cannot be used as a source. The reflections from PSP are very clear beneath Kii Peninsula and are still visible under the northern Kinki district at depth of about 50-70km, where no earthquakes occur. The reflectors seem to indicate aseismic slab in the northern Kinki district. A clear parallel reflectors associated with PSP are seen beneath Kii Peninsula. The lower reflector seems to coincident with the earthquake distribution, which has been considered as the plate boundary. The upper reflector is determined to be about 8-10km shallower than the lower one. Low frequency earthquakes and/or tremors occur at 30-40km deep in the upper reflector, which is less distinct at the low frequency events. The layer between the two reflectors is thought to be the oceanic crust subducting in the mantle. In the inland area, many clear reflections were obtained at depth of about 15 and 25km. These reflectors decline towards north, which seem to relate with the large active faults.
DE: 7200 SEISMOLOGY
DE: 7203 Body waves
DE: 7205 Continental crust (1219)
DE: 7218 Lithosphere (1236)
DE: 8123 Dynamics: seismotectonics
SC: Seismology [S]
MN: Fall Meeting 2005