Seismic Swarm S20020721.1: Activity Near Olancha, California

Seismic swarm S20020721.1 was recorded 11 km south of Olancha, California, beginning at 17:32 on 20 July 2002 and concluding at 14:59 on 24 July 2002. The sequence lasted 93 hours and 26 minutes and included 49 earthquakes. All events were of low to moderate magnitude, with the largest reaching 2.8. Depths remained shallow, predominantly between 1 km and 7 km, consistent with activity along local fault structures in the Owens Valley.

The swarm exhibited a typical pattern of clustered, low-magnitude events without a dominant mainshock. Initial activity on 20 July included events of magnitude 2.0 and 2.8 at depths of 4 km and 5 km. Subsequent days featured numerous events below magnitude 2.0, with occasional spikes such as magnitude 2.5 and 2.2 on 21 July. Activity gradually declined by 24 July, ending with two events of magnitude 0.6 and 0.9. Negative depth values recorded for two events likely reflect minor location uncertainties common in dense swarm catalogs.

The Olancha region lies within the Owens Valley fault zone, part of the Eastern California Shear Zone. This area accommodates dextral shear between the Pacific and North American plates at rates of several millimeters per year. Normal and strike-slip faults cut through Quaternary sediments and bedrock, producing shallow seismicity. The 1872 Owens Valley earthquake of approximately magnitude 7.4 remains the largest historical event in the immediate vicinity and demonstrates the potential for larger ruptures along the same fault system.

Since 2000, ten swarms have been documented in the area, with six occurring in 2000 and four in 2001. Swarm S20020721.1 fits this established pattern of episodic, clustered microseismicity that releases strain without producing significant ground rupture.

References

USGS Earthquake Catalog
California Geological Survey Fault Activity Map
SeismoSight internal swarm classification records