Seismic Swarm S20000126.1: Analysis of Activity Near Inyokern, California

On 25 January 2000 at 21:08 UTC, a seismic swarm designated S20000126.1 began approximately 22 km west-northwest of Inyokern, California. The sequence concluded at 05:39 on 27 January 2000, encompassing 31 earthquakes over a period of 32 hours and 30 minutes. All events were recorded at shallow depths between 2 km and 9 km, with the majority clustered around 7–9 km. Magnitudes remained modest, ranging from 0.0 to a peak of 3.6, consistent with typical swarm behavior rather than a mainshock-aftershock sequence.

The swarm initiated with two events of magnitude 1.2 and 1.3 within the first hour. Activity continued at low levels through the night before intensifying on 26 January around 11:00, when a magnitude 3.6 earthquake occurred at 7 km depth. This event was followed within minutes by several smaller shocks, including magnitudes 1.9 and 2.1. Subsequent events through the afternoon and evening maintained magnitudes below 2.5, with the final recorded shock of magnitude 1.0 closing the sequence early on 27 January.

The Inyokern region lies within the western Mojave Desert, part of the Eastern California Shear Zone. This zone accommodates a significant portion of Pacific–North American plate motion through distributed right-lateral strike-slip faulting. Local structures include segments of the Garlock Fault to the south and numerous smaller northwest-trending faults that cut through the Indian Wells Valley. The area’s geology features Quaternary alluvium overlying Mesozoic granitic basement rocks of the Sierra Nevada batholith, with thin sedimentary layers from ancient lake beds. Historical seismicity in the vicinity includes moderate events associated with the 1992 Landers and 1999 Hector Mine earthquakes, which triggered widespread aftershocks across the shear zone.

Seismic swarms in this setting commonly arise from fluid migration or aseismic slip along immature fault networks. Depths recorded during S20000126.1 align with the brittle–ductile transition zone in the region, where temperatures permit brittle failure at shallow levels. No surface rupture was associated with the swarm, and maximum magnitudes remained well below thresholds that would produce felt shaking beyond a few kilometers.

Monitoring by regional seismic networks continues to track similar low-level clusters, providing data on strain accumulation along the shear zone. Such sequences contribute to refined hazard models for the greater Los Angeles Basin and Central Valley corridors.

References

USGS Earthquake Catalog
California Geological Survey Fault Database
Southern California Earthquake Data Center