Seismic Swarm S20150510.1: Analysis of Activity Near Lee Vining, California

Seismic swarm S20150510.1 occurred in a tectonically dynamic region 27 km east-northeast of Lee Vining, California, within Mono County. This area sits at the intersection of the Sierra Nevada range and the Basin and Range province, where crustal extension drives frequent seismic events. Proximity to the Long Valley Caldera and the Mono-Inyo Craters volcanic chain adds volcanic influences to the primarily tectonic setting, with fault systems such as the Mono Lake fault contributing to localized strain release.

The swarm initiated at 09:32 on 9 May 2015 and concluded at 12:41 on 11 May 2015, spanning 51 hours and 9 minutes. During this period, 35 earthquakes were recorded. Event magnitudes ranged from 0.5 to 2.7, with the largest shock measuring 2.7 at a depth of 5 km on 9 May at 21:41. Most events clustered at depths between 4 km and 8 km, though several reached 13–15 km, indicating activity across multiple crustal levels. Smaller events dominated the sequence, reflecting typical swarm behavior where energy dissipates through numerous minor releases rather than a single mainshock-aftershock pattern.

Notable activity included a cluster on 9 May with events of 2.1, 2.0, and 2.7, followed by sustained lower-magnitude occurrences on 10 May. The final events on 11 May registered 1.6 and 0.8. Depths varied modestly, suggesting fluid migration or stress adjustments along pre-existing fractures common in this volcanic-tectonic transition zone.

Since 2000, the region has experienced 10 documented swarms, underscoring recurrent seismic patterns. These occurred in 2001 (two swarms), 2004 (two), 2006 (one), 2008 (two), 2009 (one), 2011 (one), and 2015 (one, the subject of this analysis). Such episodes often align with broader deformation in the Walker Lane belt, where right-lateral shear accommodates Pacific-North American plate motion.

This swarm exemplifies the area's moderate seismicity, posing minimal surface hazard given the low magnitudes but providing data on subsurface processes near active volcanic features. Ongoing monitoring by regional networks supports hazard assessment in this geologically complex setting.

References

USGS Earthquake Catalog (earthquake.usgs.gov)
California Geological Survey regional reports on Mono County tectonics
Long Valley Caldera Observatory summaries on seismic-volcanic interactions