Surface deformation and earthquake swarms are correlated in space and time with operations at the Brawley geothermal field in southern California. The seismicity culminated in 2012, about 2 years after the onset of geothermal activity, with a M5.4 earthquake. These earthquakes occurred at a >5km depth, much larger than the ∼1km reach of the geothermal wells, raising questions about the triggering mechanism. Surface deformation shows that aseismic slip on a normal fault intersecting the geothermal reservoir preceded the swarm and possibly triggered it. In this study, we resort to geomechanical modeling to investigate how the sequence of aseismic and seismic slip unfolded. The modeling accounts for thermo- and poro-elastic stress changes induced by the geothermal operations and allows for inelastic deformation and faulting of the reservoir and surrounding medium. The simulation successfully reproduces the flow rates and well-head pressures reported by the operator as well as the measured surface subsidence. By varying the model parameters, we show that the surface subsidence is due to thermal contraction and normal faulting. The fault reactivation is driven by pressure changes and thermal unclamping. The pressure-driven reactivation is rapid and influences a larger area, while the temperature-driven reactivation is more gradual and more localized near the injection wells. In our simulation, aseismic normal faulting driven by the geothermal operation leads to elastic stress release via yielding and faulting within the reservoir volume and, conversely, to stress build-up beneath the reservoir, where the 2012 swarm developed. Such a stress transfer provides a plausible explanation for the 2012 Brawley swarm. Our study shows how a geothermal operation can, in principle, contribute to seismic hazard mitigation through the aseismic release of tectonic stresses within a geothermal field but points to the difficulty of mitigating the hazard posed by stress transfers in the surrounding area.

地表变形和地震群在空间和时间上与加利福尼亚州南部布劳利地热田的作业相关。地震活动在 2012 年达到高潮，大约在地热活动开始后 2 年，发生了 M5.4 地震。这些地震发生在 > 5 公里的深度，远大于地热井约 1 公里的范围，引发了对触发机制的质疑。地表变形表明，与地热储层相交的正断层上的地震滑动先于蜂群，并可能触发了它。在这项研究中，我们采用地质力学建模来研究地震和地震滑动的序列是如何展开的。该建模考虑了由地热操作引起的热弹性和多孔弹性应力变化，并考虑到储层和周围介质的非弹性变形和断层。模拟成功地再现了操作员报告的流速和井口压力以及测量的地面沉降。通过改变模型参数，我们表明地表下沉是由于热收缩和正常断层造成的。故障重新激活是由压力变化和热松开驱动的。压力驱动的再活化是快速的并且影响更大的区域，而温度驱动的再活化更渐进并且更集中在注入井附近。在我们的模拟中，由地热操作驱动的抗震正断层导致通过储层体积内的屈服和断层产生弹性应力释放，相反，在储层下方形成应力，2012 年群在此形成。这种压力转移为 2012 年布劳利群提供了一个合理的解释。我们的研究表明，地热作业原则上如何通过地热场内构造应力的抗震释放来减轻地震灾害，但指出减轻周围地区应力转移造成的灾害的难度。