Study on the internal structure of seismic wave reflectors beneath the source regions of inland earthquakes

在东北大学地震台网附近的地震台记录的浅部内陆地震的垂直和横向分量地震图中，偶尔观察到明显的后期震相。我们将这些后期阶段解释为PxP和SXS波在地壳反射层的同一平面上反射。该反射体位于1998年长町-立府断层最深处的5.0级地震震源区下方。该反射体的深度约为13km，其走向和倾角与5.0级地震断层面的走向和倾角基本一致。不同的反射体也分别位于栗驹山和田泽湖下，靠近日本东北部弧的火山前缘，深度分别为1.3km和1.11km。根据反射波与直达波的谱振幅比，估计了反射体中的纵、横波速度结构及其厚度。假设反射体外部的地震波速度和Q值为适当的值，通过网格搜索改变反射体的厚度、地震波速度和Q值等参数来估计反射体内部结构的最佳拟合模型。在这三个区域中，反射体的厚度估计约为100 m。反射体中的P波和S波速度分别为周围地壳中P波和S波速度的40 <$45%和29 <$43%。假设反射体由具有随机取向和分布的球状夹杂物的复合材料模拟，我们通过一种新的自洽方案计算了复合材料的有效弹性常数。在这三个区域观测到的P波和S波速度表明，反射体中的裂缝中充满了水，而不是部分熔融的材料。

Distinct later phases were occasionally observed both in vertical and transverse component seismograms of shallow inland earthquakes recorded at nearby seismic stations of Tohoku University network. We interpreted these later phases as PxP and SxS waves reflected at the same plane of the reflectors in the crust. The reflector was located beneath the focal area of 1998 M5.0 earthquake, which occurred at the deepest portion of the Nagamachi-Rifu fault. The depth of the reflector is 〜 13 km, and the strike and dip-angle of the reflector are almost the same as those of the M5.0 earthquake fault plane. Distinct reflectors were also located at a depth of 〜3 km and 〜11 km beneath Mt.Kurikoma and Lake Tazawa close to the volcanic front in the northeastern Japan arc. We estimated P-and S-wave velocity structure in the reflector bodies and their thickness from spectral amplitude ratios of reflected waves to direct waves. Assuming appropriate values for seismic wave velocity and Q-value outside of the reflector bodies, the bestfit model for the internal structure of the reflector bodies was estimated by grid search varying parameters such as thickness, seismic wave velocity and Q-value of the reflector. The thicknesses of reflector bodies are estimated to be about 100 m in the three regions. P wave and S wave velocities in the reflector bodies are estimated to be 40〜45% and 29〜43% of those in the surrounding crust, respectively. Assuming that the reflector body is modeled by a composite material having randomly oriented and distributed spheroidal inclusions, we calculated the effective elastic constants of the composite by a new self-consistent scheme. Observed P and S wave velocities in the three regions suggest that cracks in the reflector bodies are filled with water rather than partial molten materials.