Seismic Swarm S20030417.1: Eastern Sierra Nevada, California

Seismic swarm S20030417.1 was recorded 21 km east-northeast of Sunny Slopes in Mono County, California. The sequence began at 07:04 on 17 April 2003 and concluded at 15:35 on 18 April 2003, spanning 32 hours and 30 minutes. During this interval, 27 earthquakes were detected. Magnitudes ranged from 0.0 to 1.9. The largest event measured 1.9 and occurred at 19:32 on 17 April at a depth of 4 km. Most events clustered between 6 km and 8 km depth, with a few shallower or deeper outliers reaching 0 km and 11 km. Activity was concentrated in the first 12 hours, after which event frequency declined steadily until termination. This swarm represents the second documented episode in the area since 1 January 2000. The preceding swarm occurred in 2000. Both sequences share similar characteristics of low-magnitude, closely spaced events without a dominant mainshock. The region lies within the western Basin and Range province, where northwest-directed extension accommodates motion between the Sierra Nevada block and the North American plate. Sunny Slopes sits near the southwestern margin of Long Valley Caldera, a 17-by-32 km volcanic depression formed approximately 760,000 years ago during a major rhyolitic eruption. Subsequent Holocene activity includes smaller explosive events and persistent geothermal circulation driven by a residual magma body at 5–10 km depth. Earthquake swarms in this setting commonly arise from fluid migration along pre-existing faults or from pressure changes within the shallow hydrothermal system. Depths recorded during S20030417.1 align with the brittle-ductile transition zone influenced by elevated geothermal gradients. Regional faults, including segments of the Hartley Springs and Hilton Creek systems, exhibit Quaternary offset and continue to accommodate minor extension. Instrumental monitoring by the USGS and regional networks has documented recurrent swarms since the 1980s unrest episode. These events typically last hours to days and rarely exceed magnitude 3.0. No surface rupture or significant ground deformation has been associated with S20030417.1, consistent with the low energy release. Ongoing volcanic and tectonic processes maintain elevated seismicity rates relative to most of California. Continued surveillance supports hazard assessment for nearby communities and infrastructure, including U.S. Route 395 and the town of Mammoth Lakes.

References

SeismoSight internal swarm classification records.
USGS Earthquake Hazards Program catalog and Long Valley Caldera monitoring reports.
California Geological Survey Quaternary fault database.