HR: 0800h
AN: S51B-0998    [Abstracts]
TI: Determination of Focal Mechanisms of Microearthquakes and Estimation of the Stress Field in the Tanba Region in Central Japan
AU: * OGASAWARA, T
EM: oga@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji Kyoto, 611-0011 Japan
AU: KATAO, H
EM: katao@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji Kyoto, 611-0011 Japan
AU: IIO, Y
EM: iio@rcep.dpri.kyoto-u.ac.jp
AF: Disaster Prevention Research Institute, Kyoto University, Gokasho, Uji Kyoto, 611-0011 Japan
AB: It is important that we examine mechanism of microearthquakes when we estimate occurrence mechanism of large earthquake and regional stress field. In the Tanba region, northern Osaka prefecture and middle Kyoto prefecture in Japan, the microearthquake activity that are not aftershocks of a large earthquake is active constantly. Katao et al. (1997) determined focal mechanisms of some microearthquakes of the Tanba region in the process of determining mechanisms of aftershocks of the Hyogo-ken Nanbu earthquake. However, determination of mechanisms is not done routinely in this area. In this study, we estimated stress changes caused by the 1995 Hyogo-ken Nanbu earthquake at the neighboring area, as seen in the P-axis directions by doing detailed analyses for small region. We manually read P-wave onset polarities, and determined focal mechanisms using the method of Maeda (1992) for about 800 events larger than M2.0 during 1995-1999. In these events, we used about 400 events as good solutions that have a P-axis error within 10 degrees. About the period before the Hyogo-ken Nanbu earthquake, we used data of focal mechanisms determined by Iio (1996). From the data mentioned above, we examined time and space distribution of P-axis directions. We used also a method of Horiuchi (1995) to estimate suitable principle stress field and stress ratio, because we have to study more quantitatively about stress field in this area. It is known that P-axis directions are generally E-W around the Tanba region. We estimated principal stress field using the method of Horiuchi (1995), the result shows that the stress field is consistent with P-axis direction. However, after the Hyogo-ken Nanbu earthquake, there are small areas with differences in a P-axis direction. After the Hyogo-ken Nanbu earthquake, many atypical earthquakes occurred in the Tanba region. Such a change of P-axis directions is evident particularly in two years after the Hyogo-ken Nanbu earthquake.
DE: 7200 SEISMOLOGY
DE: 7215 Earthquake source observations (1240)
DE: 7218 Lithosphere (1236)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7294 Seismic instruments and networks (0935, 3025)
SC: Seismology [S]
MN: Fall Meeting 2005