Seismic Analysis of the 2001 Anza Earthquake Swarm

The October-November 2001 earthquake swarm near Anza, California, designated Swarm S20011031.1, provides a clear example of clustered seismic activity along the San Jacinto Fault Zone. The sequence began at 05:28 on 31 October 2001 and concluded at 22:34 on 10 November 2001. It was centered 17 km east-southeast of Anza in Riverside County and produced 471 earthquakes over 257 hours and 6 minutes.

The Anza region lies within the San Jacinto Fault Zone, one of Southern California’s most active fault systems. This zone accommodates a significant portion of the dextral shear between the Pacific and North American plates. The local geology consists of Mesozoic metamorphic and granitic rocks of the Peninsular Ranges, cut by northwest-trending faults. The Anza Gap segment, located immediately northwest of the swarm epicenters, has not produced a major surface-rupturing earthquake in historical time and remains a focus of seismic-hazard studies.

Examination of the first 100 events reveals a classic swarm pattern initiated by a magnitude 5.0 earthquake at 07:56 on 31 October at 13 km depth. This event was followed within minutes by numerous smaller shocks whose magnitudes clustered between 0.5 and 2.5. Focal depths for the majority of these events ranged from 13 to 16 km, consistent with the brittle-ductile transition zone along the San Jacinto Fault. A secondary peak magnitude of 2.8 occurred later on the same day at 15 km depth. Magnitudes declined rapidly after the initial hours, with most subsequent events below magnitude 2.0. The temporal distribution shows intense activity in the first 12 hours, followed by a gradual decay, typical of swarm behavior rather than a classic mainshock-aftershock sequence.

Depths remained remarkably uniform, suggesting that the swarm occurred on a single fault patch at mid-crustal levels. No events shallower than 5 km appear in the initial 100 records, indicating limited shallow brittle failure. The spatial concentration of epicenters within a few kilometers of the initial magnitude 5.0 event implies a compact source volume.

This swarm fits the long-term pattern of moderate-magnitude clusters observed along the San Jacinto Fault Zone. Historical records document similar sequences in 1968, 1980, and 1999, all characterized by rapid onset, short duration, and maximum magnitudes between 4.5 and 5.5. Such activity releases strain without producing the large, damaging earthquakes expected from the locked Anza Gap segment.

Current monitoring by the Southern California Seismic Network continues to track microseismicity in the area. Updated fault models incorporate geodetic data showing ongoing interseismic strain accumulation northwest of the 2001 swarm locus. These observations reinforce the importance of continued surveillance for any acceleration in seismic rates that might signal larger rupture potential.

References

• Southern California Earthquake Data Center, event catalog 2001
• USGS Quaternary Fault and Fold Database, San Jacinto Fault Zone
• SCEC Community Fault Model, version 5.3