HR: 0800h
AN: T51C-0698    [Abstracts]
TI: Fine fault structure in intraplate earthquake region estimated by DD method with waveform correlation analysis, using nationwide seismic network Hi-net, Japan
AU: * Yukutake, Y
EM: yukutake@bosai.go.jp
AF: National research Institute for Earth science and Disaster prevention, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Takeda, T
EM: ttakeda@bosai.go.jp
AF: National research Institute for Earth science and Disaster prevention, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National research Institute for Earth science and Disaster prevention, 3-1 Tennodai, Tsukuba, 305-0006, Japan
AB: Determining highly resolved hypocenter distribution in intraplate earthquake region is very important for estimation of fine fault structure. The dense permanent nationwide seismic observation network (NIED Hi-net) with an average spacing of 20-30km has been developed all over the Japan region after the disasters of 1995 Kobe earthquake. The detection capability and accuracy of hypocentral location was quite improved by Hi-net. On the other hand, a double-difference (DD) earthquake location algorithm (Waldhauser and Ellsworth, 2000) was developed to improve the accuracy of relative hypocenter location. In this method, further improvement of location is expected by using waveform cross-correlation method. We applied DD algorithm using waveform cross- correlation method to the Hi-net data, to estimate the fine fault structure and confirm the availability of Hi-net routine seismic observation network. We relocated the hypocenters of aftershock sequences of two moderate crustal earthquakes, and of seismicity around the active fault system which displays one of the largest slip rates in the Japanese islands. We used the absolute travel time measurements and differential travel time measurements by waveform cross-correlation analysis. In this analysis, the velocity waveform of 0.75 sec containing the P or S wave onset was used, applying 3-20 Hz band-pass filter. Moderate crustal earthquakes occurred in the northern part of Mie Prefecture on 15 April, 2007 (Mw=5.0) and in the southwest part of Shizuoka Prefecture on 1 June, 2007 (Mw=4.4), each of which followed by aftershock sequences. In both regions, the hypocenter of aftershocks determined by routine process are scattered and we could not find any trend of main shock fault. However, the locations by DD method reveal the fine structure of seismicity. Most of events around the main shock hypocenter show a planer distribution, which is consistent with the focal mechanism of main shock. This planer distribution of hypocenters probably reflects the structure of main shock fault. In the northern part of Mie Prefecture, the locations by DD method also reveal another cluster 1 km off from the main shock hypocenter, which is parallel to the planer distribution of hypocenters around the main shock. This lineament might suggest a subsidiary fault system parallel to the fault of main shock. We relocated the hypocenter location around the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), which has large slip rate. In the northern part of ISTL, we found the planer distribution of hypocenters under the ISTL. In the central and south part of ISTL, on the other hand, the hypocenter distributions form a cloud surrounding the ISTL, which suggest that most of events did not occur on the ISTL in these areas. We concluded that the fine fault structure concerned with the moderate crustal earthquakes and the active fault system could be resolved from the hypocentral distribution by Hi-net, applying DD method. Aftershock sequences of moderate earthquakes define the planer distribution of hypocenters, which suggest that most of aftershocks concentrate on the fault of main shock.
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8010 Fractures and faults
DE: 8015 Local crustal structure
SC: Tectonophysics [T]
MN: 2007 Fall Meeting