HR: 1340h
AN: S13A-1039    [Abstracts]
TI: Spatial-temporal variation of b-value around the aftershock area of 2000 Western Tottori earthquake, Japan.
AU: * Kato, A
EM: akato@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Nakagawa, S
EM: shigeki@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi , Ibaraki pref., 305-0006 Japan
AU: Iidaka, T
EM: iidaka@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Yoshida, S
EM: shingo@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, Univ. of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AB: Recently, several publications have documented the spatial and temporal variations in the frequency-magnitude distribution in various crustal faults. Many factors can cause the perturbations of the normal {\it b} value: enhancement of material heterogeneity, an increase in applied shear stress, decreases the {\it b} value [e.g., Wiemer and Wyss, 1997]. Since many factors influence the frequency-magnitude distribution, it may not always be clear what perturbs the {\it b} value. Therefore, it is crucial to investigate the {\it b} value anomalies of crustal faults by compare the other known parameters (e.g., slip distributions during the mainshock, seismicity) in the crust. In this paper, we have calculated the spatial distributions of {\it b} values at two different time periods after a large earthquake in the western Tottori prefecture of Japan, and discussed the spatial-temporal variations of {\it b} value. On October 6, 2000, an earthquake with JMA magnitude of 7.3 was occurred in the western Tottori prefecture of Japan. The Joint Group for the Dense Aftershock Observation carried out a dense aftershock observation in and around the source region at 2-month period in 2000 after the mainshock [Shibutani et al., 2004], and 1-year period in 2003 [Katao et al., 2002]. To investigate the aftershock distributions, we have applied the joint hypocenter determination (JHD) [Kissling et al., 1994] to the picked data obtained from a new auto-pick algorithm [Chiba et al, 2002], and then the hypoDD method has been conducted [Waldhauser and Ellsworth, 2000]. We estimated the {\it b} value at every nodal point of 1 km separation, using the 200 nearest earthquakes within the concentrated aftershock zone. The {\it b} value is calculated by the maximum likelihood method. At l-st observation period just after the mainshock, the high {\it b} values are located at the shallow portions, and the low {\it b} values seem to surround the high {\it b} value area. This high {\it b} value region coincides with the location of the area with highest slip release during the mainshock determined form the strong motion data [Wu, 2004]. We plot the frequency-magnitude distribution for two individual regions, and the difference between high-{\it b} and low-{\it b} values is clear and is statistically significant. This result supports the hypothesis that the highest slip area released the shear stress during the mainshock, and the released stress was redistributed to the surrounding area. At 2-nd observation period (three years later after the mainshock), the area with low {\it b} value in 1-st period shows the increase in {\it b} value, and the two {\it b} values are different at the 95% confidence limit. In contrast, {\it b} value in the area, where high {\it b} value is observed at the 1-st period, slightly increases. These temporal variations of {\it b} values suggest that the stress released to the surrounding area during the mainshock has been relaxed, and the high slip region during the mainshock has might accumulate the stress as the preparatory process of a future earthquake.
DE: 8164 Stresses--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
DE: 7215 Earthquake parameters
DE: 1734 Seismology
SC: Seismology [S]
MN: 2004 AGU Fall Meeting