Seismic Swarm S20021024.3 Near Anza, California

The seismic swarm S20021024.3 was recorded 10 km east-southeast of Anza, California. Activity began at 22:21 on 23 October 2002 and concluded at 15:57 on 26 October 2002. Over 65 hours and 36 minutes, 38 earthquakes were detected.

This swarm occurred within the San Jacinto Fault Zone, a major right-lateral strike-slip system forming part of the boundary between the Pacific and North American tectonic plates. The zone accommodates significant dextral shear and has produced multiple moderate to large earthquakes historically. Depths of the recorded events ranged primarily between 10 km and 15 km, consistent with the brittle-ductile transition typical of the regional crust.

Key events included a magnitude 3.8 earthquake at 17:18 on 24 October at 13 km depth, followed by a cluster of events between magnitudes 1.4 and 2.4 within the subsequent 15 minutes. Earlier activity featured smaller events (magnitudes 0.7–1.5) at depths of 10–14 km. Later shocks on 25 and 26 October remained below magnitude 2.2 and occurred at similar depths, with one shallower event at 6 km.

Swarm sequences in this area often reflect fluid migration or aseismic slip triggering brittle failure along fault segments. The 2002 swarm fits patterns observed in the Anza seismic gap, where strain accumulation occurs without frequent large ruptures.

Historical statistics since 1 January 2000 indicate only two swarms in the immediate region. The prior swarm occurred in 2001 (one event cluster), followed by this 2002 sequence.

The Anza region has long been recognized for elevated microseismicity due to its position along the San Jacinto Fault. Paleoseismic studies document recurrent surface-rupturing events over the Holocene, underscoring the zone's capacity for significant earthquakes.

References

• USGS Earthquake Catalog (queried for Anza, CA region, 2002 events)
• Southern California Seismic Network reports on San Jacinto Fault Zone
• SeismoSight internal swarm classification records