5:15 PM - 6:45 PM
[SCG55-P09] The background stress in the aftershock area of the 2000 Tottori Western earthquake estimated from stress rotation
Keywords:stress rotation, stress orientation, principal stress axis, directional statistics, von Mises distribution, Kagan angle
Iwata [2018, JGR] proposed a Bayesian approach to estimating a spatial stress pattern from P-wave first motions. The proposed approach was applied to the aftershock area of the 2000 Western Tottori earthquake. Iwata [2023, JpGU Meeting] modified the proposed approach to improve the spatial resolution of the estimation; the Delaunay triangulation was incorporated to represent the spatial stress pattern. The modified approach was also applied to the same dataset as the one Iwata [2018] analyzed, and clarified the stress orientation (stress rotation) difference across the main fault on both the north and south edges of the area where a large slip occurred.
In this study, the background stress in the aftershock area of the 2000 Tottori Western earthquake was estimated using the stress rotation as described above. For simplicity, it was assumed that the orientations of the maximum and minimum principal stress axes of the background stress are EW and NS, respectively. Therefore, the orientation of the intermediate principal stress axis is assumed to be vertical. The coseismic stress distribution was calculated using the slip model of Iwata et al. [2000, SSJ Fall Meeting] and the program of Okada [1992, BSSA]. Then, a particular value of the differential stress was given, and the spatial pattern of the principal stress axes was computed from the sum of the background stress and the coseismic stress. The value of the differential stress was optimized to fit the computed spatial pattern with the one estimated from Iwata [2023] as much as possible; the difference between the two stress patterns was measured with the Kagan angle [Kagan, 1991, GJI], and the optimization was conducted through the maximum likelihood method where the Kagan angle is assumed to follow the von Mises distribution. The optimal differential stress was estimated at 9 MPa.
In this study, the background stress in the aftershock area of the 2000 Tottori Western earthquake was estimated using the stress rotation as described above. For simplicity, it was assumed that the orientations of the maximum and minimum principal stress axes of the background stress are EW and NS, respectively. Therefore, the orientation of the intermediate principal stress axis is assumed to be vertical. The coseismic stress distribution was calculated using the slip model of Iwata et al. [2000, SSJ Fall Meeting] and the program of Okada [1992, BSSA]. Then, a particular value of the differential stress was given, and the spatial pattern of the principal stress axes was computed from the sum of the background stress and the coseismic stress. The value of the differential stress was optimized to fit the computed spatial pattern with the one estimated from Iwata [2023] as much as possible; the difference between the two stress patterns was measured with the Kagan angle [Kagan, 1991, GJI], and the optimization was conducted through the maximum likelihood method where the Kagan angle is assumed to follow the von Mises distribution. The optimal differential stress was estimated at 9 MPa.