Seismic Swarm S20220419.1: Analysis of Activity Near Anza, California

Seismic swarm S20220419.1 was recorded 8 km southwest of Anza, California, in the San Jacinto Fault Zone. The sequence began at 04:55 on 18 April 2022 and concluded at 06:43 on 26 April 2022, spanning 193 hours and 47 minutes with a total of 76 earthquakes.

The events were predominantly microseismic, with magnitudes ranging from 0.0 to 2.0. Depths varied between 2 km and 17 km, indicating activity across shallow crustal levels typical of the region. Early activity on 18 April featured numerous events below magnitude 1.0 at depths of 2–8 km. Subsequent days showed a gradual increase in both frequency and occasional larger events, including several magnitude 1.8–2.0 shocks between 22 and 25 April at depths of 10–16 km. The sequence remained tightly clustered in both space and time, consistent with swarm behavior rather than a classic mainshock-aftershock pattern.

The Anza area lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that forms part of the boundary between the Pacific and North American plates. This zone accommodates a significant portion of the relative plate motion in southern California and is characterized by high seismicity rates, frequent earthquake swarms, and occasional moderate events. The local geology consists of Mesozoic crystalline basement rocks overlain by Quaternary sediments in the valleys, with the fault zone exhibiting complex segmentation that influences rupture behavior.

Historical records indicate that the region has hosted repeated swarms. Since 2000, 39 swarms have been documented in the immediate vicinity, with notable increases in frequency during 2017–2021. These sequences typically involve hundreds of small events over days to weeks and are thought to result from fluid migration or aseismic slip transients along the fault. The 2022 swarm fits this established pattern, contributing to the ongoing monitoring of strain accumulation in the Anza seismic gap, a section of the fault that has not produced a major rupture in over 200 years.

No damage or felt reports were associated with the swarm, reflecting the low magnitudes involved. Continued observation of such sequences provides valuable data on fault zone processes and helps refine seismic hazard assessments for the densely populated areas of southern California.