Seismic Swarm S20050612.1: Analysis of Activity Near Anza, California

Seismic swarm S20050612.1 occurred in a tectonically active zone 14 km east-southeast of Anza, California. The sequence initiated at 11:34 on 12 June 2005 and concluded at 07:28 on 9 July 2005, spanning 643 hours and 53 minutes. During this interval, 904 earthquakes were recorded, characteristic of swarm behavior where events cluster in time and space without a single dominant mainshock-aftershock pattern.

The first 100 events provide key insight into the swarm's onset. Activity began with a magnitude 0.7 event at 8 km depth. Approximately four hours later, a magnitude 5.2 earthquake occurred at 13 km depth, followed by numerous smaller aftershocks. Subsequent events ranged primarily from magnitude 0.2 to 2.6, with the majority clustered between 1.0 and 2.0. Depths concentrated between 11 and 15 km, though isolated readings extended to 17 km or registered negative values likely due to location uncertainties. This distribution indicates brittle failure within the mid-crustal seismogenic zone, consistent with strike-slip faulting prevalent in the region.

The Anza area lies within the Peninsular Ranges province of southern California, where the San Jacinto Fault Zone accommodates a significant portion of the Pacific-North American plate boundary slip. This right-lateral strike-slip system trends northwest-southeast and intersects with subsidiary structures that facilitate swarm activity. Historical records since 2000 show five prior swarms in the immediate vicinity, occurring in 2001 (one swarm), 2002 (two swarms), and 2003 (two swarms). These recurrent episodes suggest episodic fluid migration or stress triggering along fault segments with varying frictional properties.

Geological mapping and geophysical studies confirm that the local crust comprises Mesozoic granitic and metamorphic rocks overlain by thin sedimentary cover. Seismicity is driven by the broader transform boundary dynamics, with the nearby San Andreas Fault contributing to regional strain accumulation. Depths observed in the swarm align with the typical 10–15 km brittle-ductile transition in this part of the fault system.

Analysis of the initial events reveals a rapid increase in rate following the magnitude 5.2 shock, with events distributed across a compact volume. Magnitudes remained modest after the initial peak, supporting classification as a swarm rather than a classic foreshock-mainshock sequence. Such patterns are documented in other California fault zones where aseismic slip or pore-pressure changes promote clustered microseismicity.

Continued monitoring of the Anza region remains essential given its proximity to populated areas and infrastructure. The 2005 swarm underscores the value of dense seismic networks in resolving fine-scale fault processes and improving hazard assessment for the San Jacinto Fault Zone.

References

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