The 2020 Westmorland, California Earthquake Swarm as Aftershocks of a Slow Slip Event Sustained by Fluid Flow

Swarms are bursts of earthquakes without an obvious mainshock. Some have been observed to be associated with transient aseismic fault slip, while others are thought to be related to fluids. However, the association is rarely quantitative due to insufficient data quality. We use high‐quality GPS/GNSS, InSAR, and relocated seismicity to study a swarm of >2,000 earthquakes which occurred between 30 September and 6 October 2020, near Westmorland, California. Using 5 min sampled Global Positioning System (GPS) supplemented with InSAR, we document a spontaneous shallow Mw 5.2 slow slip event that preceded the swarm by 2–15 hr. The earthquakes in the early phase were predominantly non‐interacting and driven primarily by the slow slip event resulting in a nonlinear expansion. A stress‐driven model based on the rate‐and‐state friction successfully explains the overall spatial and temporal evolution of earthquakes, including the time lag between the onset of the slow slip event and the swarm. Later, a distinct back front and a square root of time expansion of clustered seismicity on en‐echelon fault structures suggest that fluids helped sustain the swarm. Static stress triggering analysis using Coulomb stress and statistics of interevent times suggest that 45%–65% of seismicity was driven by the slow slip event, 10%–35% by inter‐earthquake interactions, and 10%–30% by fluids. Our model also provides constraints on the friction parameter and the pore pressure and suggests that this swarm behaved like an aftershock sequence but with the mainshock replaced by the slow slip event.

群震是指没有明显主震的一系列地震。据观察，有些群震与短暂的无震断层滑动有关，而另一些则被认为与流体有关。然而，由于数据质量不足，这种关联很少能定量确定。我们利用高质量的全球定位系统（GPS）/全球导航卫星系统（GNSS）、合成孔径雷达干涉测量（InSAR）以及重新定位的地震活动数据，对2020年9月30日至10月6日发生在加利福尼亚州威斯特摩兰附近的一次超过2000次地震的群震进行研究。利用5分钟采样的全球定位系统（GPS）并辅以InSAR，我们记录到一次自发的浅源5.2级慢滑事件，它比群震提前2 - 15小时发生。早期阶段的地震主要是非相互作用的，并且主要由慢滑事件驱动，导致非线性扩展。一个基于速率 - 状态摩擦的应力驱动模型成功地解释了地震的整体时空演化，包括慢滑事件开始和群震之间的时间滞后。后来，在雁列断层结构上出现了明显的后沿以及聚类地震活动的时间平方根扩展，这表明流体有助于维持群震。利用库仑应力进行的静态应力触发分析以及事件间时间的统计表明，45% - 65%的地震活动是由慢滑事件驱动的，10% - 35%是由地震间相互作用驱动的，10% - 30%是由流体驱动的。我们的模型还对摩擦参数和孔隙压力提供了约束，并表明这个群震的行为类似于余震序列，但主震被慢滑事件所取代。