Research on earthquakes, crustal movements and groundwater discharge changes in the Rokko active fault region

在位于1995年兵库县南部地震（M=7.2）震源区内的观测站（Rokko-Takao、Rokko-Syakunage和Rokko-Turukabuto）进行了地壳应变和倾斜的连续观测。六子-高尾断裂和六子-鹤嘴断裂的观测隧道分别与满浦地断裂和大月断裂交叉。在六子-高尾地区，还测量了满浦库吉断裂断裂带的地下水流量变化。在这项工作中，在地震发生后继续进行观测，并对数据进行分析。在六子鹤埠站观测到地震引起的最大应变变化为400μ应变，最大高度变化为8.7mm。在地震前很长一段时间内，没有发现短期变化或震前的变化，但应变速率缓慢下降，最大应变方向的小旋转和约0.1mm/年的相对剧变。地震发生后，大月断层开始以0.1 ~ 0.2mm/年的速度进行横向和纵向错位。位错与第四纪断层的运动意义相同。同震阶梯状地壳变形后，可观测到具有较长时间常数的震后地壳变形。作为引起这种变形的原因，我们考虑了震源附近的后滑和地壳对同震滑动的粘弹性响应。在本研究中，我们认为同震应变变化产生了与应变变化相对应的同震孔隙压力变化，地震后地壳变形是由地下水从高压区向包括地表在内的低压区迁移引起的。运移速率可能受这些区域的渗透率控制。

Continuous observations of crustal strains and tilts have been executed in the stations (Rokko-Takao, Rokko-Syakunage and Rokko-Turukabuto), which are located within the focal region of the 1995 Hyogo-ken Nanbu earthquake (M=7.2). The observation tunnels of the Rokko-Takao and Rokko-Tsurukabuto are crossing with the Manpukuji and Otsuki faults, respectively. In the Rokko-Takao, change in groundwater discharge from the fracture zone of the Manpukuji fault has beeu also measured. In this work, the observations were continued after the earthquake, and the data analysis was done.At the Rokko-Tsurukabuto station, a maximum strain change of 400μ strain and a maximum height change of 8.7mm caused by the earthquake were observed. Short-term changes or changes immediately before the earthquake were not detected, but a slow decrease of strain rates, small rotations of the maximum strain directions and a relative upheaval of about 0.1mm/year continued for a long time prior to the earthquake. After the earthquake, the Otsuki fault started lateral and vertical dislocations of a rate of 0.1-0.2mm/year. The dislocations are the same sense as the Quaternary movements of the fault.A post-seismic crustal deformation which has a comparatively long time constant might be observed after a step-like coseismic crustal deformation. As causes where such deformatian is caused, an after-slip in the vicinity of a hypocenter, and a visco-elastic response of the crust to coseismic slipping have been considered. In present research, we consider that the coseismic strain changes generate the coseismic pore pressure changes which corresponds to the strain changes and that the postseismic crustal deformation is caused by groundwater migrations from the high pressure areas to the low pressure areas including the ground surface. The migration rates might be controlled by permeability of those areas.