Seismic Swarm S20200509.2: Analysis Near Coso Junction, California

Seismic swarm S20200509.2 was recorded 11 km northeast of Coso Junction, California. The sequence began at 19:10 on 8 May 2020 and concluded at 18:48 on 12 May 2020, encompassing 49 earthquakes over 95 hours and 38 minutes. Magnitudes ranged from -0.1 to 2.3, with the largest event occurring at 15:53 on 9 May 2020 at a depth of 3 km. Most events clustered between 2 km and 6 km depth, consistent with shallow crustal processes in the region.

The swarm exhibited typical characteristics of clustered microseismicity, featuring numerous low-magnitude events interspersed with occasional larger shocks. Early activity on 8 May included events of magnitude 0.9–1.0 at depths of 3–6 km. Activity intensified on 9 May, with multiple events above magnitude 1.0, including a 1.5 at 10:17 and the peak 2.3 later that afternoon. Subsequent days showed declining frequency but persistent small events through the final recorded shock of magnitude 0.7 on 12 May.

Historical records indicate 89 swarms in the area since 1 January 2000. Annual counts include 4 in 2000, 4 in 2001, 2 in 2002, 8 in 2004, 2 in 2005, 6 in 2006, 1 in 2009, 7 in 2010, 1 in 2011, 4 in 2012, 6 in 2013, 2 in 2014, 2 in 2015, 4 in 2016, 2 in 2017, 1 in 2018, 23 in 2019, and 10 in 2020. This pattern reflects recurrent seismic episodes.

The Coso region lies within the Coso Volcanic Field, an area of Quaternary volcanism situated in the western Basin and Range province of eastern California. The field features rhyolitic domes, basaltic flows, and extensive geothermal systems driven by shallow magmatic heat sources. Tectonic extension combined with hydrothermal fluid circulation produces frequent earthquake swarms. Depths recorded in this swarm align with the brittle-ductile transition zone influenced by elevated geothermal gradients.

Such swarms are common in geothermal fields worldwide and are often linked to fluid migration rather than magmatic intrusion. The Coso Geothermal Power Plant, one of the largest in the United States, operates within this setting and routinely monitors microseismicity for operational safety.

References

USGS Earthquake Hazards Program catalog data for California.
California Geological Survey regional geologic maps of the Coso Volcanic Field.
Peer-reviewed literature on Basin and Range extension and geothermal seismicity (e.g., studies published in Journal of Geophysical Research).