Seismic Activity: The January 2002 Earthquake Swarm Near Borrego Springs, California
The region 15 km NNW of Borrego Springs, California, lies within the tectonically active San Jacinto Fault Zone, part of the broader San Andreas Fault system. This area experiences frequent seismicity due to right-lateral strike-slip motion along the boundary between the Pacific and North American plates. The local geology features sedimentary basins, crystalline basement rocks, and alluvial deposits shaped by ongoing faulting and erosion in the Anza-Borrego Desert.
Earthquake swarms are common in this zone, often linked to fluid migration or stress transfer along fault segments. Historical records indicate low overall swarm frequency, with only one documented swarm since January 2000—the event of 2001. The S20020102.1 swarm began at 07:56 on 2 January 2002 and concluded at 13:50 on 5 January 2002, lasting 77 hours and 54 minutes. It produced 53 earthquakes, with hypocentral depths predominantly between 7 km and 13 km.
The sequence initiated with a magnitude 0.9 event at 10 km depth. The largest shock, magnitude 4.2 at 11 km depth, occurred at 12:11 on 2 January, followed by an aftershock sequence of smaller events mostly below magnitude 2.0. Depths remained stable around 10–13 km for the majority of events, with occasional shallower occurrences near 6–9 km later in the swarm. Magnitudes clustered tightly between 0.7 and 1.9 after the initial peak, showing a typical swarm decay pattern without a clear mainshock-aftershock cascade.
This activity reflects the region's persistent background seismicity, where swarms serve as indicators of fault-zone dynamics rather than precursors to larger quakes. Updated monitoring since 2002 has confirmed continued low-level activity along the same fault segments, consistent with long-term strain accumulation rates of several millimeters per year.
References
SeismoSight internal classification for swarm S20020102.1
USGS Earthquake Hazards Program regional reports on San Jacinto Fault Zone seismicity