15:30 〜 16:30
[J05-P-10] The spatial distribution of the stress ratio in the aftershock area of the 2000 Western Tottori Earthquake
A statistical approach to estimate a spatial stress pattern from P-wave first motions has been developed. In this approach, without the determination of focal mechanisms, the spatial pattern of stress tensor is evaluated under the two assumptions: (i) the orientation of a fault plane is distributed randomly and uniformly; and (ii) the slip orientation of faulting is parallel to the one in which the shear stress is maximized. With the two assumptions and smoothness constraint on the spatial variation in the stress tensor, the best estimate of the spatial stress pattern to fit observed P-wave first motions is found in the framework of a Bayesian statistics.
This approach was applied to the dataset of P-wave first motions taken from the aftershocks of the 2000 Western Tottori Earthquake. From the dataset compiled by Kawanishi et al. [2009, JGR], the P-wave first motions with which O-C time for the P-wave arrival is within 0.1 s and that for the S-wave arrival is within 0.2 s (if the S-wave arrival is picked) were selected. After this selection, 47,570 P-wave first motions for 3592 events were remained and analyzed.
From the estimated stress tensor, the spatial pattern of the stress ratio R = (sigma_1-sigma_2)/(sigma_1-sigma_3) was calculated where sigma_1, sigma_2, and sigma_3 denote the maximum, intermediate, and minimum principal stress, respectively. In the southern part of the main fault orientated NNW-SSE, the value of R along the western side of the fault is high while it is low along the eastern side. The Tottori earthquake has a left-lateral strike slip fault, and therefore we may assume the stress field after the mainshock is compressive and extensional along the western and eastern sides along the southern part of the main fault, respectively. The spatial pattern of the stress ratio revealed in this study is consistent with the assumed stress field.
This approach was applied to the dataset of P-wave first motions taken from the aftershocks of the 2000 Western Tottori Earthquake. From the dataset compiled by Kawanishi et al. [2009, JGR], the P-wave first motions with which O-C time for the P-wave arrival is within 0.1 s and that for the S-wave arrival is within 0.2 s (if the S-wave arrival is picked) were selected. After this selection, 47,570 P-wave first motions for 3592 events were remained and analyzed.
From the estimated stress tensor, the spatial pattern of the stress ratio R = (sigma_1-sigma_2)/(sigma_1-sigma_3) was calculated where sigma_1, sigma_2, and sigma_3 denote the maximum, intermediate, and minimum principal stress, respectively. In the southern part of the main fault orientated NNW-SSE, the value of R along the western side of the fault is high while it is low along the eastern side. The Tottori earthquake has a left-lateral strike slip fault, and therefore we may assume the stress field after the mainshock is compressive and extensional along the western and eastern sides along the southern part of the main fault, respectively. The spatial pattern of the stress ratio revealed in this study is consistent with the assumed stress field.