Earthquake Swarm S20181207.1 Near Bodie, California

An earthquake swarm designated S20181207.1 occurred approximately 20 km southeast of Bodie, California, in Mono County. The sequence began at 05:32 UTC on 7 December 2018 and concluded at 08:01 UTC on 8 December 2018, spanning 26 hours and 29 minutes. During this interval, 33 earthquakes were recorded, with magnitudes ranging from 0.8 to 1.9 and focal depths between 2 km and 14 km.

The events clustered tightly in both space and time, a hallmark of swarm activity where no single mainshock dominates. The largest events reached magnitude 1.9 at depths of 6 km and 9 km on 7 and 8 December, respectively. Shallower events (depths of 2–5 km) occurred primarily in the later stages of the sequence, including several magnitude 1.4 shocks at 2 km depth on 7 December. This vertical distribution suggests fluid migration or localized stress adjustments within the upper crust.

The Bodie region lies within the Walker Lane belt, a northwest-trending zone of dextral shear accommodating approximately 20–25 % of the Pacific–North America plate motion. The belt transects the western Basin and Range province and exhibits elevated seismicity driven by both strike-slip and normal faulting. Historical mining activity in the Bodie Hills exposed extensive volcanic and volcaniclastic rocks of Miocene age, but contemporary seismicity is primarily tectonic rather than volcanic in origin.

Since 1 January 2000, eighteen swarms have been documented in the immediate vicinity. These episodes occurred in 2001 (two), 2004 (two), 2006 (one), 2008 (two), 2009 (one), 2011 (one), 2015 (two), 2016 (two), 2017 (four), and 2018 (one). The recurrence pattern indicates persistent, low-level strain release along distributed faults rather than large, infrequent events.

Analysis of the 2018 swarm reveals a typical swarm signature: rapid onset, lack of a clear magnitude–time decay, and predominance of small-magnitude events. Depths mostly between 7 km and 13 km align with the seismogenic zone in this part of the Walker Lane. The absence of felt reports or damage is consistent with the modest magnitudes involved.

Ongoing monitoring by regional seismic networks continues to track microseismicity in the area, contributing to improved understanding of strain accumulation along the California–Nevada border. Such swarms provide valuable data for refining fault models and assessing background hazard levels in this tectonically active corridor.

References

SeismoSight internal swarm catalog S20181207.1
USGS Earthquake Hazards Program regional seismicity reports
California Geological Survey, Walker Lane tectonic framework publications