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Detection of a healing process of the Nojima fault by a repeated water injection experiment and understanding of the mechanism of induced earthquakes

通过重复注水实验检测野岛断层的愈合过程并了解诱发地震的机制

基本信息

批准号：
11304025
负责人：
SHIMAZAKI Kunihiko
金额：
$ 11.58万
依托单位：
University of Tokyo
依托单位国家：
日本
项目类别：
Grant-in-Aid for Scientific Research (A).
财政年份：
1999
资助国家：
日本
起止时间：
1999 至 2000
项目状态：
已结题

项目摘要

A water injection experiment was carried out at the 1800m-deep borehole at the Nojima fault where the 1995 Kobe earthquake took place, in January to March 2000, three years after the first experiment conducted two years after the Kobe earthquake. The migration of the injected water was monitored by various geophysical measurements at the borehole and two other boreholes nearby, the seismic observation, and self-potential and resistivity measurements of the earth. The result of these observations and measurements suggests that the permeability in the ground surrounding the 1800m-deep borehole and also in a shallow (at depths of 0-4km) fracture zone of the Nojima fault has decreased, which is most likely to indicate a healing process after the Kobe earthquake. Although our new temperature measurement revealed that the injected water leaked at a depth of 540m, most of the injected water appears to migrate towards the fracture zone of the Nojima fault. Changes in water discharge at the bottom of a 800m-deep borehole, horizontally 50m away from the 1800m borehole, indicate that the permeability of surrounding rocks has decreased to about one-half that measured three years ago. The decrease of the permeability of surrounding rocks was also confirmed by self-potential observation. The activity of ultra-microearthquake in a distance of 2-4km from the borehole, increased four to seven days after the start of water injection, which is delayed two days in comparison with the previous experiment. If the activity was triggered by the injection of the water, the delay indicates a decrease in the permeability within the fracture zone of the fault. The migration of seismic sources and b-value decrease suggest that the seismic activity was induced by the water injection.

在1995年科比地震发生后的第一次注水实验进行了两年，2000年1月至3月，在1800米深的钻孔中进行了注水实验。注入水的迁移通过钻孔和附近的另外两个钻孔的各种地球物理测量、地震观测以及大地的自然电位和电阻率测量来监测。这些观察和测量的结果表明，在1800米深的钻孔周围的地面，以及在浅（在0- 4公里的深度）的野岛断层断裂带的渗透性已经降低，这是最有可能表明愈合过程后，科比地震。虽然我们新的温度测量显示注入水在540米的深度泄漏，但大部分注入水似乎向野岛断层的断裂带迁移。800 m深钻孔底部（距1800 m钻孔水平50 m处）排水量的变化表明，围岩渗透性已下降到三年前的一半左右。围岩渗透性的降低也被自电位观测所证实。注水后4 ~ 7天，距井眼2-4km范围内的超微地震活动增强，比上次实验推迟了2天。如果活动是由水的注入触发的，则延迟指示断层的断裂带内的渗透性的降低。震源的迁移和b值的降低表明地震活动是由注水引起的。