Seismic Swarm S20190825.1 Near Ridgecrest, California
Seismic swarm S20190825.1 was recorded 23 km north of Ridgecrest, California, beginning at 20:14 on 24 August 2019 and concluding at 03:24 on 28 August 2019. Over 79 hours and 10 minutes, the swarm produced 165 earthquakes. The events occurred in a tectonically active portion of the Mojave Desert within the Eastern California Shear Zone, a region characterized by distributed right-lateral strike-slip faulting that accommodates part of the Pacific–North America plate boundary motion.
Analysis of the first 100 events reveals predominantly low-magnitude activity. Recorded magnitudes ranged from 0.1 to 2.6, with the majority below 1.0. Depths were shallow, concentrated between 2 km and 10 km, consistent with brittle failure in the upper crust. Notable events within this subset included a magnitude 2.6 earthquake at 21:31 on 26 August at 3 km depth, followed closely by a magnitude 2.4 event at the same depth. Earlier activity featured several magnitude 1.5–1.9 shocks distributed across depths of 3–9 km. The temporal distribution showed clustering, with increased rates during the afternoon and evening of 25 August and early hours of 26 August.
This swarm fits into a broader pattern of seismic activity in the Ridgecrest area. Since 1 January 2000, 39 swarms have been documented in the region. Yearly counts include single swarms in 2000, 2005, 2009, 2011, 2014, 2017, and 2018; four swarms each in 2006, 2010, and 2013; two swarms in 2012, 2015, and 2016; and 14 swarms in 2019. The elevated 2019 total reflects heightened regional seismicity following the Ridgecrest Earthquake Sequence.
A magnitude 7.1 mainshock on 6 July 2019, part of that sequence, occurred only 4 km from the center of swarm S20190825.1. The July event ruptured multiple fault segments and was preceded by a magnitude 6.4 foreshock on 4 July. These ruptures highlighted the interconnected nature of faults in the shear zone, where stress transfer can trigger both large events and subsequent swarms.
The geological setting features Quaternary alluvial fans and playas overlying crystalline basement rocks cut by northwest-trending faults. Ongoing dextral shear produces frequent small earthquakes, with swarms often linked to fluid migration or aseismic slip on minor structures. Depths observed in S20190825.1 align with the typical seismogenic zone thickness of 10–15 km in this part of the Mojave.
References
SeismoSight internal classification data for swarm parameters and event lists.
USGS Earthquake Hazards Program catalog for regional tectonic framework and 2019 Ridgecrest sequence details.