Seismic Swarm S20200411.1: Analysis of Activity Southeast of Bodie, California

An earthquake swarm designated S20200411.1 was recorded in the region 30 km southeast of Bodie, California. The sequence began at 14:36 on 11 April 2020 and concluded at 22:40 on 14 May 2020. Over approximately 800 hours and 3 minutes, a total of 1019 earthquakes were registered.

The swarm unfolded in the Bodie Hills area of Mono County, part of the Basin and Range province in eastern California. This tectonic setting features active normal faulting and Quaternary volcanism, with the local geology dominated by Miocene to Pliocene andesitic and dacitic flows, tuffs, and breccias overlying Mesozoic basement rocks. Seismic activity in the vicinity is influenced by regional extension along the Walker Lane belt and proximity to volcanic centers such as the Mono-Inyo Craters system.

Examination of the first 100 events reveals an initial mainshock of magnitude 5.2 at 8 km depth on 11 April 2020 at 14:36:37 UTC. Subsequent activity consisted primarily of events between magnitude 1.0 and 4.5, with the majority clustered in the 1.4–2.8 range. Depths predominantly ranged from 0 to 10 km, though a few negative depth values appear in the record. Notable later shocks in this subset included magnitudes of 4.5 at 8 km, 3.9 and 3.7 at 7 km, and several events of 3.0–3.5. The temporal distribution showed the highest rate of occurrence in the first several hours, gradually declining thereafter while maintaining a diffuse spatial pattern consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Historical records indicate that 20 swarms have occurred in the same area since 1 January 2000. These episodes were distributed across multiple years, with the highest counts in 2017 (four swarms) and 2018 (three swarms). Earlier activity included two swarms each in 2001, 2004, 2008, 2015, and 2016, plus single swarms in 2006, 2009, and 2011.

Such recurrent swarms are typical of the region’s hydrothermal and volcanic environment, where fluid migration and minor magmatic movement can trigger prolonged clusters of small-to-moderate earthquakes without producing surface rupture. Continued monitoring by regional seismic networks remains essential for distinguishing background tectonic activity from potential precursors to larger volcanic or hydrothermal events.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey, Bodie Hills geologic mapping reports
SeismoSight internal swarm classification records