HR: 11:05h
AN: S32B-04    [Abstracts]
TI: Estimation of Fault Strength Before the 1995 Kobe Earthquake
AU: * Yamashita, F
EM: yamafuto@bosai.go.jp
AF: Nat'l Res. Inst. Earth Sci. Disas. Prev., Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Fukuyama, E
EM: fuku@bosai.go.jp
AF: Nat'l Res. Inst. Earth Sci. Disas. Prev., Tennodai 3-1, Tsukuba, 305-0006 Japan
AU: Omura, K
EM: omura@bosai.go.jp
AF: Nat'l Res. Inst. Earth Sci. Disas. Prev., Tennodai 3-1, Tsukuba, 305-0006 Japan
AB: We propose a new method to estimate the strength of a fault. The most direct way to measure fault strength is to observe the stress field just before an earthquake. However, it is difficult to monitor in-situ stress continuously, making the observation of pre-shock stress nearly impossible. Instead, we have reconstructed the stress field before the 1995 Kobe earthquake ({\it M}$_{w}$6.9) using in-situ post-shock stress measurements by hydraulic fracturing experiments and a kinematic source model. The in-situ stress measurements were taken about one year after the earthquake at four sites near the fault (Ikeda {\it et al}., 2001; Tsukahara {\it et al}., 2001). The background stress field in this region was considered to be east-west compression from the focal mechanisms of small earthquakes (Katao {\it et al}., 1997). After the earthquake, however, the maximum horizontal stress direction around the fault was estimated to be northwest-southeast, which is perpendicular to the fault, from in-situ stress measurements and the mechanisms of aftershocks. This rotation of the principal stress direction was due to the fault slip of the earthquake. We estimated the pre-shock stress field by subtracting the stress change due to the coseismic slip from the post-shock stress field. For a kinematic source model, we selected Yoshida's model inverted from geodetic and seismic data (Yoshida {\it et al}., 1996). The estimated stress state shows that at the center of the fault, the maximum principal stress direction is east-west, which is consistent with the background stress before the earthquake. Moreover, the horizontal differential stress was estimated to be large: linear gradient of the differential stress was estimated to be 19 MPa/km. On the contrary, at both edges of the fault, the stress field did not change due to the earthquake and its principal direction was perpendicular to the fault. After the estimation of the pre-shock stress field, we were then able to estimate the coefficient of static friction using the shear and normal stress on the fault plane. At each depth, pore pressure is assumed to be hydrostatic. The average coefficient of static friction at the center of the fault was estimated to be 0.60, which is consistent with Byerlee's law in the laboratory (Byerlee, 1978). The strength of the fault is found to be equivalent to the strong crust. On the other hand, the coefficients at the edge of the fault were estimated to be very small (0-0.25), which suggests that the faults in those regions could not sustain the shear stress and an aseismic slip might occur even before the earthquake.
DE: 7260 Theory and modeling
DE: 8123 Dynamics, seismotectonics
DE: 8164 Stresses--crust and lithosphere
DE: 7209 Earthquake dynamics and mechanics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting