Seismic Swarm S20160331.1 Near Coso Junction, California

Seismic swarm S20160331.1 occurred approximately 16 km east of Coso Junction, California, beginning at 08:35 on 30 March 2016 and concluding at 23:31 on 31 March 2016. Over 38 hours and 56 minutes, the event sequence comprised 27 earthquakes. Magnitudes ranged from 0.0 to 2.4, with the largest events recorded at 2.4 and 2.3. Depths were predominantly shallow, clustered between 1 km and 2 km. The sequence initiated with smaller events around magnitude 1.5–1.6, escalated to moderate peaks midday on 30 March, and featured a secondary cluster of events above magnitude 2.0 late on 31 March before tapering off.

This swarm reflects typical behavior in the Coso region, where clustered microseismicity often occurs without a single dominant mainshock. Most events remained below magnitude 2.0, consistent with low-level energy release distributed across multiple small ruptures.

The Coso Junction area lies within the Coso Volcanic Field in eastern California, part of the western Basin and Range province. The region experiences active extensional tectonics driven by the interaction between the Pacific and North American plates, producing north-northwest-trending normal faults. Geothermal activity is prominent due to shallow magmatic heat sources beneath rhyolite domes and associated fracture networks. The Coso Geothermal Field, one of the largest in the United States, has operated since the 1980s, with fluid extraction and injection known to influence local seismicity patterns.

Seismic swarms have been recurrent in this setting. Historical records since 2000 document 49 such swarms through 2015, distributed across multiple years with notable concentrations in 2004 (7 swarms), 2006 (6), 2010 (7), 2013 (6), and 2012 (4). These episodes are attributed to a combination of tectonic strain accumulation, hydrothermal fluid migration, and minor magmatic influences at depth.

The 2016 swarm fits established patterns, with events concentrated at shallow crustal levels where geothermal reservoirs and fault intersections facilitate rapid stress transfer. Depths of 1–2 km align with the brittle-ductile transition zone influenced by elevated geothermal gradients in the volcanic field.

References

• United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey, Regional Geologic Maps
• Coso Operating Company geothermal field reports and monitoring summaries