HR: 0800h
AN: S41A-0968    [Abstracts]
TI: Applications of Natural Earthquake Reflection Profiling Method
AU: * Nakagawa, S
EM: nakagawa@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hirata, N
S41A-0968 AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Obara, K
S41A-0968 AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AU: Kasahara, K
S41A-0968 AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AU: Ito, Y
S41A-0968 AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006 Japan
AU: Kato, A
S41A-0968 AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Kurashimo, E
S41A-0968 AF: Earthquake Research Institute, the University of Tokyo, Japan, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: A common midpoint (CMP) reflection method is widely used to image the heterogeneous crustal structures in active seismic profiling. The sources and receivers are located on the Earth's surface. The controlled sources used in the CMP method are, however, limited both in magnitudes and in the number of sources. Furthermore, it is generally not efficient to use S wave energy in controlled sources. Using the natural earthquakes at depth in the crust as the sources in the seismic reflection profiling method may improve crustal images, as compared to the conventional controlled source method. The advantages of using natural earthquakes include following: the distance between the source and the reflector may be short, the energy of the sources is larger than that of the controlled source, and S wave energy is easy to use. We developed a method, the Natural Earthquake Reflection Profiling (NERP) method, to image the reflectors using natural earthquakes [Nakagawa et al., 2005]. In this study, we applied this method to the natural earthquake data in three regions, 1) western Tottori prefecture, 2) Kii peninsula, and 3) southern Taiwan. The specifications of earthquake observations in these regions are as follows. 1) Western Tottori prefecture: On October 6, 2000, a large earthquake (Mw 6.6) occurred in the western Tottori prefecture, Japan. Although this earthquake is a large intra-island-arc type event, no surface rupture was clearly observed. To image the crustal structure in the fault zone, a multi-channel aftershock observation was carried out. A multi-channel seismic (MCS) array was deployed along and across the main fault area. The length of MCS array was 12 km. Each channel was equipped with nine vertical-component seismometers, each with a natural frequency of 10 Hz. The array consisted of 240 channels with receiver spacing of 50 m. The signal was recorded with the GDAPS-4 digital telemetry system at a sampling rate of 4 ms. The MCS array was operated 85 hours to obtain quasi-continuous records of aftershocks, and 269 events were recorded. 2) Kii peninsula: The Philippine Sea plate is subducted beneath the Eurasian plate from the Nankai trough and large thrust earthquakes have occurred with an interval of 100 years. In southwest Japan including southern Kii peninsula, non-volcanic deep tremors have been detected [Obara, 2002]. To reveal the crustal structure of this area, several kinds of seismic observations were carried out. Two MCS arrays were deployed at southern Kii peninsula. Each channel was equipped with three component seismometer with natural frequency of 1 Hz or vertical component seismometer with a natural frequency of 2 Hz. The array consisted of 120 channels with receiver spacing of 20 or 30 m. The signal was recorded with the StrataVisor digital telemetry system at a sampling rate of 8 ms. The MCS array was operated one and half months, and about 1500 events were recorded. 3) Southern Taiwan: Hirata et al. (2005) conducted the natural earthquake observation in southern Taiwan to clarify the crustal and upper mantle structure. A MCS array was deployed at southern Taiwan. Each channel was equipped with vertical component seismometer with a natural frequency of 4.5 Hz. The array consisted of 90 channels with receiver spacing of 100 m. The signal was recorded with the MS2000 digital recorders at a sampling rate of 4 ms. The MCS array was operated one month, and about 30 events were recorded. The application of NERP method to those data shows many clear reflectors in the crust, and has a potential to understand the tectonics of each area.
DE: 0935 Seismic methods (3025, 7294)
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8038 Regional crustal structure
SC: Seismology [S]
MN: Fall Meeting 2005