Seismic Swarm S20010301.1 Near Arvin, California: A Detailed Review of the 2001 Event

Seismic swarm S20010301.1 occurred approximately 20 km northeast of Arvin in Kern County, California. The sequence began at 11:57 on 28 February 2001 and concluded at 08:43 on 6 March 2001, spanning 140 hours and 46 minutes. During this period, 74 earthquakes were recorded, with magnitudes ranging from 0.8 to 2.2 and focal depths predominantly between 0 and 8 km.

The swarm exhibited typical characteristics of clustered seismic activity, with events concentrated in the initial days before tapering off. On 28 February alone, 21 earthquakes occurred, including the largest event of magnitude 2.2 at 14:34. Subsequent days showed reduced frequency, with notable clusters on 1 March (15 events) and 2–3 March. Depths clustered around 4 km for the majority of events, suggesting activity within shallow crustal layers, while a few registrations indicated depths at or near the surface.

Geologically, the region lies at the transition between the southern San Joaquin Valley and the Tehachapi Mountains. This setting is influenced by the broader San Andreas Fault system and nearby structures such as the Garlock Fault, which accommodate right-lateral strike-slip motion driven by Pacific–North American plate interaction. The underlying geology features sedimentary basins overlying basement rocks, with active deformation contributing to regional seismicity. Historical records document significant events in the area, including the 1952 Kern County earthquake of magnitude 7.3, which highlighted the potential for larger ruptures along local faults.

Analysis of the swarm reveals no dominant mainshock-aftershock pattern; instead, the energy release was distributed across numerous small events. This distribution is consistent with fluid migration or stress triggering in a tectonically active zone. Most events remained below magnitude 2.0, posing minimal surface impact but providing valuable data on local fault behavior.

The sequence concluded without escalation, underscoring the often benign nature of swarms in this part of California. Ongoing monitoring by regional networks continues to refine understanding of such phenomena in the context of long-term seismic hazard assessment.

References

United States Geological Survey Earthquake Catalog California Geological Survey Regional Fault Maps SeismoSight Internal Swarm Classification Records