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

在1995年兵库县南部7.2级地震震源区内的六号站(六号站、六号站和图鲁卡布托站)进行了地壳应变和倾斜的连续观测。Rokko-Takao和Rokko-Tsurukabuto的观测隧道分别与Manpukuji和Otsuki断层相交。在Rokko-Takao，还测量了满浦吉断裂断裂带地下水流量的变化。震后继续观测，并进行数据分析，在Rokko-Tsurukabuto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8.7 mm。地震前没有发现短期的变化或变化，但地震前应变速率的缓慢下降、最大应变方向的小旋转和约0.1毫米/年的相对隆起持续了很长一段时间。地震发生后，大树断裂开始以每年0.1-0.2毫米的速度发生侧向和垂直错动。位错与断裂的第四纪运动具有相同的意义，在阶梯状同震形变之后，可能会观察到具有较长时间常数的震后地壳形变。作为引起这种变形的原因，考虑了震源附近的余滑和地壳对同震滑动的粘弹性响应。在目前的研究中，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压变化，震后地壳形变是地下水从高压区向包括地表在内的低压区的迁移所致。运移速率可能受控于这些地区的渗透率。

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.