Abstract The stress field of the Earth's crust is spatially heterogeneous on various scales. The focal mechanism of an earthquake provides important information associated with the stress state at the seismogenic depths. These focal mechanisms are affected by the pore fluid pressure and/or friction coefficient. In this study, we focused on misfit angles, particularly the difference in the angle between the slip direction in an observed focal mechanism and the resolved shear stress direction that can be obtained using the directions of the principal stress axes and the stress ratio of a reference stress field. Resolved shear stress does not depend on the fault friction or pore fluid pressure, allowing us to use misfit angles to evaluate the heterogeneity of stress fields if the effect of an error in a focal mechanism has been properly removed. By statistically evaluating the error in focal mechanisms using a Bayesian approach, we evaluated the misfit angles of the aftershocks of the 2000 Western Tottori Earthquake, in which numerous focal mechanisms could be precisely determined using dense seismic observation. The spatial distribution of the misfit angles of the aftershocks suggested that the stress field in the aftershock region, particularly in the southern portion of the region, was in a heterogeneous state. We also demonstrated that this spatial variability cannot be explained solely by errors in the focal mechanisms. If we assume a uniform stress field before the mainshock, the observed cumulative curve of misfit angles can be explained by coseismic stress changes and the background stress field's depth gradient of differential stress of 3.2 MPa/km.

中文翻译：

---

使用聚焦机制中的失配角估计应力场的不均匀性

摘要 地壳应力场在不同尺度上具有空间异质性。地震的震源机制提供了与发震深度应力状态相关的重要信息。这些震源机制受孔隙流体压力和/或摩擦系数的影响。在这项研究中，我们专注于失配角，特别是观察到的震源机制中的滑移方向与可使用主应力轴方向和参考应力比获得的解析剪应力方向之间的角度差异应力场。解析剪应力不依赖于断层摩擦力或孔隙流体压力，如果震源机制中的误差影响已被适当消除，我们可以使用失配角来评估应力场的不均匀性。通过使用贝叶斯方法统计评估震源机制的误差，我们评估了 2000 年西鸟取地震余震的错配角度，其中使用密集地震观测可以精确确定众多震源机制。余震错配角的空间分布表明，余震区，特别是该区南部的应力场处于非均质状态。我们还证明，这种空间变异性不能仅由焦点机制中的错误来解释。如果假设主震前应力场均匀，则观测到的错配角累积曲线可以用同震应力变化和背景应力场的3.2 MPa/km微分应力深度梯度来解释。