Group A, Poster #069, Tectonic Geodesy

The 2020 Westmorland, California earthquake swarm as aftershocks of a slow slip event sustained by fluid flow

Krittanon Sirorattanakul, Zachary E. Ross, Mostafa Khoshmanesh, Elizabeth S. Cochran, Mateo Acosta, & Jean-Philippe Avouac
Poster Image: 

Poster Presentation

2022 SCEC Annual Meeting, Poster #069, SCEC Contribution #12106 VIEW PDF
Swarms are bursts of earthquakes without an obvious mainshock. Some have been observed to be associated with aseismic transients, 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 that occurred between September 30 and October 6, 2020, near Westmorland, California. Using 5-min sampled GPS supplemented with InSAR, we document a spontaneous shallow Mw 5.19 slow slip event that preceded the swarm by approximately half a day. The earthquakes in the early phase were predominantly non-interacting and driven primar...ily by the slow slip event resulting in a non-linear 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 the en-echelon 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, 20 – 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.