Seismic Swarm S20170521.1: Analysis of Activity Near Pearsonville, California

Seismic swarm S20170521.1 was recorded in the region 26 km north-northeast of Pearsonville, California. The sequence began at 14:34 on 20 May 2017 and concluded at 06:37 on 22 May 2017, encompassing a total duration of 40 hours and 2 minutes. During this interval, 35 earthquakes were registered, with magnitudes ranging from -0.1 to 2.0 and focal depths predominantly between 1 and 5 km.

The swarm exhibited typical characteristics of clustered microseismicity, featuring numerous low-magnitude events interspersed with occasional larger shocks. Notable activity included two magnitude 2.0 events on 21 May at 02:49 and 11:07, alongside several magnitude 1.0 events clustered around midnight on 21 May. Depths remained shallow throughout, consistent with activity influenced by local crustal structures. Event frequency peaked during the evening of 20 May and early hours of 21 May before tapering toward the swarm's conclusion.

This swarm aligns with the broader seismic regime of the area, situated within the Eastern California Shear Zone. The zone accommodates a portion of the relative motion between the Pacific and North American plates through distributed right-lateral strike-slip faulting. Historical deformation in the region has produced notable surface-rupturing events, and ongoing microseismicity reflects continued strain accumulation along subsidiary faults. Proximity to volcanic and geothermal features, such as those associated with the Coso Volcanic Field, contributes to elevated background seismicity driven by fluid migration and magmatic processes.

Seismic swarms have been a recurrent feature here. Since 1 January 2000, 51 swarms have been documented. Annual counts include 4 in 2000, 4 in 2001, 2 in 2002, 7 in 2004, 2 in 2005, 6 in 2006, 1 in 2009, 7 in 2010, 1 in 2011, 4 in 2012, 4 in 2013, 2 in 2014, 3 in 2015, and 4 in 2016. These episodes underscore persistent tectonic and hydrothermal influences on local fault systems.

The 2017 swarm provides additional insight into the episodic nature of strain release in this tectonically active corridor. Shallow depths and low magnitudes indicate limited potential for significant ground shaking, yet such sequences contribute to refined understanding of fault interactions and stress transfer in the shear zone.

References

SeismoSight internal swarm classification records.
USGS Earthquake Catalog (regional tectonic framework).
California Geological Survey fault database.