The injection of water into geothermal systems is an important procedure required to recover subsurface water resources and enhance permeability for increasing the reservoir volume. The injected water often leads to microseismic events during migration, which can be used to directly track the location of the injected water. However, in rare cases, unexpectedly large induced seismicity occurs after the injection termination. For risk control, understanding the differences between cases that cause post‐termination seismicity and those that do not is necessary. For this purpose, we used microseismic monitoring to examine the behavior of water during two injection tests, including their post‐termination periods, in Okuaizu geothermal field, Japan. In this field, a new remote microseismic cluster, apart from the injection well, was created in the post‐termination period of the first injection test. However, this cluster was not well activated in the second injection test. As a result, we revealed that this microseismic cluster was created on a structure that was different from the target fracture of the injection, possibly owing to pore‐pressure migration in the post‐termination period of the first injection. Its inactivation in the second post‐termination period may be attributable to the lower magnitude of pore‐pressure migration derived from the smaller amount of injected volume compared with that of the first injection test. The lower pore‐pressure migration was insufficient to reactivate the seismicity. We concluded that the occurrence of seismicity after injection termination may depend on the magnitude of pressure in the injection well at the shut‐in time. The Kaiser effect (i.e., a fault is not reactivated by stress that is less than the maximum stress loaded previously) could explain the observed phenomena.

中文翻译：

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地震的激活与灭活：日本大水津地热田两次注入试验的终止

将水注入地热系统是恢复地下水资源和提高渗透率以增加储层容量所需的重要程序。注入的水通常在迁移过程中导致微震事件，可用于直接跟踪注入的水的位置。但是，在极少数情况下，注入终止后会产生意想不到的大感应地震活动。为了进行风险控制，必须了解引起终止后地震活动性的案例与没有引起地震后的案例之间的差异。为此，我们使用微震监测技术在日本Okuaizu地热田进行了两次注入测试（包括终止后的时期）中的水行为。在这一领域，除了注入井以外，还有一个新的远程微震群，是在第一次注射测试的终止后阶段创建的。但是，该簇在第二次注射测试中未得到很好的激活。结果，我们发现这种微震团簇是在与注入目标裂缝不同的结构上形成的，这可能是由于在第一次注入终止后的孔隙压力迁移所致。它在终止后的第二个阶段的失活可能是由于与第一次注入试验相比，由于注入体积较小而导致的孔隙压力迁移幅度较小。较低的孔隙压力迁移不足以恢复地震活动性。我们得出的结论是，注入终止后地震活动的发生可能取决于注入时井中压力的大小。