SUMMARY We investigated the induced seismicity, source mechanisms and mechanical responses of a decametre-scale hydraulic stimulation of a pre-existing shear zone in crystalline rock, at the Grimsel Test Site, Switzerland. The analysis reveals the metre-scale complexity of hydraulic stimulation, which remains hidden at the reservoir-scale. High earthquake location accuracy allowed the separation of four distinct clusters, of which three were attributed to the stimulation of fractures in the damage zone of the shear zone. The source mechanism of the larger-magnitude seismicity varied by cluster, and suggests a heterogeneous stress field already prevailing before stimulation, which is further modified during stimulation. In the course of the experiment, stress redistribution led to the aseismic initiation of a tensile-dominated fracture, which induced seismicity in the fourth of the identified seismic clusters. The streaky pattern of seismicity separated by zones without seismicity suggests fluid flow in conduits along existing fracture planes. The observed submetre scale complexity questions the forecasting ability of induced seismic hazard at the reservoir scale from small-scale experiments.

中文翻译：

---

在水力增产实验中，米级应力非均质性和应力再分布驱动复杂的裂缝滑动和裂缝扩展

总结 我们在瑞士格里姆塞尔试验场调查了结晶岩中预先存在的剪切带的 10 米级水力刺激的诱发地震活动、震源机制和机械响应。分析揭示了水力增产的米级复杂性，而这种复杂性仍然隐藏在储层规模上。高地震定位精度允许分离四个不同的集群，其中三个归因于剪切带损伤区裂缝的刺激。较大震级地震活动的震源机制因集群而异，表明在增产前已经存在异质应力场，在增产期间进一步改变了这种应力场。在实验过程中，应力重新分布导致拉伸主导断裂的抗震起始，在已识别的第四个地震群中引发了地震活动。由没有地震活动的区域分隔的地震活动的条纹模式表明，流体沿着现有的裂缝平面在管道中流动。观测到的亚米级复杂性质疑小规模实验对水库规模诱发地震灾害的预测能力。