Seismic Swarm S20190821.1: Analysis of the August 2019 Earthquake Sequence near Little Lake, California

Seismic swarm S20190821.1 was recorded in the region 20 km east-southeast of Little Lake, California. The sequence began at 21:20 on 20 August 2019 and concluded at 03:42 on 24 August 2019, encompassing 383 earthquakes over 78 hours and 21 minutes. This activity occurred within the Eastern California Shear Zone, a tectonically active belt of distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific-North America plate motion. The area features shallow crustal deformation influenced by nearby structures such as the Little Lake fault zone and proximity to the Coso Volcanic Field, where geothermal fluid circulation can modulate seismicity rates.

The swarm exhibited characteristics typical of fluid-driven or aseismic-slip-triggered sequences, with events clustered in time and space but lacking a dominant mainshock-aftershock pattern. Magnitudes remained modest throughout, and focal depths were predominantly shallow, consistent with brittle failure in the upper 15 km of crust.

Historical Context of Seismicity

Earthquake swarms have been recurrent in this portion of the Mojave Desert since at least 2000. Records indicate 63 swarms between 1 January 2000 and the end of 2019. Annual counts varied considerably: four swarms each in 2000 and 2001, one in 2002, seven in 2004, one in 2005, six in 2006, one in 2009, seven in 2010, one in 2011, four in 2012, four in 2013, one in 2014, three in 2015, four in 2016, two in 2017, one in 2018, and twelve in 2019. This distribution underscores the region’s persistent swarm-prone behavior, often linked to the interplay between regional tectonics and local hydrothermal systems.

Insights from the Initial Events

Examination of the first 100 events reveals a narrow magnitude range, with values spanning 0.0 to 2.5. The largest event reached magnitude 2.5 on 22 August at 13:16:39 UTC. Depths clustered between 0 km and 16 km, with the majority falling between 2 km and 10 km. Early activity on 20 August consisted of isolated events of magnitude 0.5–1.2 at depths of 3–8 km. Subsequent hours on 21 August produced a gradual increase in rate, including several magnitude 1.4–2.2 events at depths of 0–11 km. By 22 August the sequence showed sustained low-level productivity, with repeated events near 5–10 km depth and occasional shallower (2 km) or deeper (15 km) outliers. No clear migration of hypocenters was evident in the initial phase, suggesting distributed failure across a small fault network rather than propagation along a single structure.

Overall, the swarm remained well below thresholds that would generate felt shaking for most residents, aligning with the historical pattern of modest-magnitude clusters in this sector of the shear zone.

References

• United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey Regional Fault and Seismic Hazard Maps
• SeismoSight internal swarm classification database