Earthquake Swarm S20050820.1 Near Anza, California

An earthquake swarm designated S20050820.1 occurred 12 km east-southeast of Anza in Riverside County, California. The sequence began at 07:57 on 19 August 2005 and concluded at 07:41 on 23 August 2005, spanning 95 hours and 44 minutes. During this period, 96 earthquakes were recorded, with magnitudes ranging from 0.2 to 1.7 and focal depths primarily between 7 km and 16 km.

The swarm exhibited typical characteristics of clustered seismicity without a dominant mainshock. Early events on 19 August included several shocks near magnitude 0.5–1.7 at depths of 9–16 km. Activity continued at a steady rate through 20 and 21 August, featuring repeated events of magnitude 0.5–1.4. The largest shocks reached 1.7 on both 19 and 22 August. By 23 August, the rate declined, with final events below magnitude 1.0. Depths remained consistent in the upper to mid-crust, consistent with regional fault structures.

Anza lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that accommodates a significant portion of the Pacific–North American plate motion. The fault zone trends northwest–southeast through the Peninsular Ranges and has produced multiple historical earthquakes exceeding magnitude 6. The 12 km ESE location places the swarm on or near secondary strands or step-over structures associated with the Clark or Coyote Creek faults. Seismicity in this area often reflects distributed strain release along a geometrically complex fault network.

Since 2000, six swarms have been documented in the immediate region, occurring in 2001 (one swarm), 2002 (two swarms), 2003 (two swarms), and 2005 (this event). Such sequences are common along the San Jacinto Fault and are thought to result from aseismic slip, fluid migration, or stress triggering within the fault zone. The 2005 swarm remained at low magnitudes and did not culminate in a larger event, illustrating the frequent microseismicity that characterizes this tectonically active corridor.

References

• USGS Earthquake Catalog (queried for Anza, CA region, 2005 events)
• Southern California Seismic Network annual reports on San Jacinto Fault Zone activity
• Geological Society of America publications on Peninsular Ranges fault architecture