Study on estimation of fluctuation in stress field and its relationship on activity of fault

首先，我们开发了一种通过反演P波、SV波、SH波振幅资料和P波极性资料，同时确定许多小地震震源机制解和震源区站短周期体波振幅修正的方法。在本次反演中，我们假设震源到各测点沿射线路径的P波和S波衰减因子可以表示为震源距离、后方位角和入射角的函数。我们将该方法应用于1996年发生在日本东北部中部的九神户地震（5.9级）的170次余震。得到的震源机制解的p轴几乎都是水平的，且向东西方向，与区域应力场基本一致。其次，将体波反演应用于短周期地震仪观测到的地震波。我们使用的观测波包括P波和S波，并对地震仪的特征进行了校正。将本研究计算的矩张量解与纵波初动分布进行对比，验证了短周期体波反演的有效性。我们将该方法应用于1998年8月3日发生在日本东北部福岛县西南部的地震余震（M4.9）。这些解几乎与主震的断面解是同一类型的，但也有一些发生在余震区的边缘，是不同类型的解。

First, we have developed a method which simultaneously determines focal mechanism solutions of many small earthquakes and source-region station corrections for short-period body wave amplitudes by inverting amplitude data of P, SV, SH waves together with P wave polarity data. In this inversion, we assumed that P and S waves attenuation factors along ray paths from hypocenters to each station can be expressed as a function of hypocentral distance, back azimuth and incident angle. We applied the present method to 170 aftershocks of the 1996 Onikobe earthquake (M5.9), which occurred in the central part of northeastern Japan. P-axes of almost all the obtained focal mechanism solutions are horizontal and oriented in the east-west direction, which are almost correspond to the regional stress field.Second, we applied the body wave inversion to seismic waves observed with the short-period seismograph. We used observed waves include P and S waves which were corrected the character of seismometer. The validity of the short period body wave inversion was verified by comparing the moment tensor solution calculated by this study to the distribution of P-wave first motion. We applied the method to aftershocks of the earthquake occurred in the southwestern part of Fukushima prefecture, northeastern Japan, August 3, 1998 (M4.9). Almost of these solutions are same type like as the fault plane solution of main shock, but some, which occurred at the edge of aftershock area, are different type solutions.