Seismic Swarm S20121016.1 Near Coso Junction, California

The seismic swarm designated S20121016.1 was recorded 28 km east of Coso Junction, California. It began at 01:17 on 15 October 2012 and concluded at 08:20 on 20 October 2012, encompassing a total of 74 earthquakes over 127 hours and three minutes. This sequence represents a classic example of swarm behavior in a tectonically and volcanically active region. The Coso area forms part of the Coso Volcanic Field in eastern California, situated at the boundary between the Walker Lane shear zone and the Basin and Range province. This setting features active extensional tectonics, with normal and strike-slip faulting accommodating regional strain. The volcanic field includes Pleistocene rhyolite domes and flows, with the most recent eruptions occurring in the Holocene. Geothermal activity is intense, driven by shallow heat sources linked to cooling magma bodies, which supports commercial power production at the Coso Geothermal Field. Seismicity here frequently arises from fluid migration within fractured reservoirs and minor magmatic movements rather than large tectonic ruptures. Seismic swarms have characterized the region since systematic monitoring began. Between 1 January 2000 and the end of 2012, 33 distinct swarms were identified, distributed across multiple years with notable concentrations in 2000, 2004, 2006, and 2010. These episodes typically involve low-magnitude events clustered in time and space, often without a dominant mainshock. Swarm S20121016.1 fits this pattern, with event magnitudes ranging from -0.3 to 2.2 and focal depths concentrated between 0 and 8 km. The largest shocks reached 2.2 on 15 October, while the majority remained below magnitude 1.0. Depths were overwhelmingly shallow, consistent with processes occurring within or immediately beneath the geothermal production zone. Analysis of the sequence shows an initial burst of activity on 15 October, including multiple events above magnitude 1.0 within the first 24 hours. Subsequent days featured sporadic clusters, with activity tapering by 19–20 October. Such temporal evolution, lacking clear aftershock decay, aligns with fluid-driven triggering mechanisms common in geothermal and volcanic environments. No damage or felt reports were associated with these microearthquakes, reflecting their small sizes and remote location. This swarm underscores the persistent seismic hazard in the Coso region, where monitoring supports both geothermal operations and regional tectonic studies. Continued observation remains essential given the area's volcanic history and ongoing extension.

References

United States Geological Survey Earthquake Catalog California Geological Survey Regional Geologic Maps Coso Operating Company Geothermal Field Reports