Seismic Swarm S20131223.1: Analysis of Activity near Coso Junction, California

The earthquake swarm designated S20131223.1 occurred in the Coso Volcanic Field region of eastern California, centered approximately 15 km northwest of Coso Junction. This sequence began at 13:39 UTC on 23 December 2013 and concluded at 23:43 UTC on 28 December 2013, spanning 130 hours and 3 minutes. During this interval, 116 earthquakes were recorded, providing a detailed record of clustered seismic activity in a tectonically active area.

The Coso Volcanic Field lies within the Basin and Range Province, where extensional tectonics interact with a long-lived volcanic system. The region features rhyolitic domes, basaltic flows, and extensive geothermal manifestations driven by shallow crustal heat sources. Seismicity here frequently reflects both tectonic faulting along the Walker Lane belt and fluid migration within the geothermal reservoir. Depths recorded in the swarm ranged from -1 km to 6 km, consistent with brittle failure in the upper crust above the local brittle-ductile transition.

The sequence initiated with a magnitude 4.3 event at 13:39:26 on 23 December at a nominal depth of 0 km. Subsequent events included multiple shocks above magnitude 2.0 within the first hour, such as magnitudes 2.2, 2.0, and 2.6. The largest events clustered in the opening hours and again on 23 December evening, with magnitudes reaching 3.4, 3.1, 3.0, and 3.3. Magnitudes then declined, with the majority of later events falling below 2.0. Shallow depths predominated, suggesting involvement of near-surface fractures possibly influenced by geothermal fluids.

Inspection of the first 100 events reveals a typical swarm pattern: an abrupt onset, rapid decay in event size, and persistence of low-magnitude activity over several days. Notable repeats occurred at depths of 0 km and 3 km, with occasional deeper outliers at 4–6 km. No single mainshock-aftershock sequence dominated; instead, energy release remained distributed across dozens of comparable events.

This swarm fits into a broader pattern of episodic seismicity documented at the site. Since 1 January 2000, 34 swarms have been identified in the immediate area. Yearly counts include six in 2000, five in 2001, two in 2002, nine in 2004, one in 2005, two in 2006, three in 2010, two in 2012, and four in 2013. Such recurrence underscores the persistent mechanical and hydrothermal conditions that favor swarm-type behavior rather than isolated large earthquakes.

Geothermal production and associated fluid extraction at the nearby Coso Geothermal Field may modulate local pore pressures, contributing to the observed clustering. The volcanic field’s most recent eruptions occurred in the late Pleistocene, yet residual heat continues to sustain both surface hot springs and microseismicity. Ongoing monitoring by regional networks provides critical data for distinguishing purely tectonic signals from those potentially linked to geothermal operations.

In summary, swarm S20131223.1 exemplifies the characteristic seismic behavior of the Coso region, where repeated episodes of moderate-magnitude, shallow activity occur within a geothermally and tectonically dynamic crust. Continued observation remains essential for assessing any evolution in swarm frequency or maximum magnitudes.