Seismic Swarm S20040328.1: Analysis of Activity Near Olancha, California

A seismic swarm designated S20040328.1 occurred 18 km southwest of Olancha, California, between 04:22 UTC on 28 March 2004 and 04:25 UTC on 1 April 2004. In 96 hours and 2 minutes, the sequence produced 71 earthquakes. The largest event reached magnitude 3.8 at a depth of 5 km on 28 March at 07:20 UTC. Depths for the full sequence ranged primarily from 3 km to 10 km, with one anomalous reading at –1 km likely reflecting location uncertainty. Magnitudes clustered between 0.5 and 3.1, consistent with swarm behavior lacking a single dominant mainshock.

The events were tightly clustered in both space and time. Early activity on 28 March included a rapid succession of events between magnitudes 1.3 and 3.8 within the first few hours. Subsequent days showed continued low-to-moderate energy release, with notable events of M2.8 on 28 March and M3.1 on 31 March. The swarm concluded abruptly after the final recorded event on 1 April.

This sequence fits the regional pattern of swarm activity documented since 2000. Eleven swarms have occurred in the area, with five in 2000, four in 2001, and two in 2002. Such recurrent swarms indicate ongoing strain release along distributed faults without producing large mainshock-aftershock sequences.

The Olancha region lies within the Eastern California Shear Zone, a network of active strike-slip and normal faults that accommodates approximately 20–25 % of Pacific–North America relative plate motion. The area is bounded by the Sierra Nevada frontal fault system to the west and the Owens Valley Fault to the east. Crustal extension and right-lateral shear produce frequent small-magnitude seismicity. Historical precedent includes the 1872 Owens Valley earthquake (estimated M 7.4–7.9), which ruptured the Owens Valley Fault and caused widespread surface offsets.

Seismic swarms in this tectonic setting commonly arise from fluid migration or aseismic slip transients rather than magmatic processes, given the distance from major volcanic centers such as Long Valley Caldera. Depths of 3–10 km align with the brittle–ductile transition zone where such mechanisms are effective.

Continued monitoring remains important because the shear zone has produced damaging earthquakes in the past and hosts multiple active fault strands capable of future moderate-to-large events.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
California Geological Survey Fault Activity Map
USGS Professional Paper on the 1872 Owens Valley Earthquake