Seismic Swarm S20200523.1 Near Redlands, California

A seismic swarm designated S20200523.1 began at 21:45 UTC on 22 May 2020 and ended at 13:08 UTC on 25 May 2020, 1 km east-southeast of Redlands, California. Over 63 hours and 23 minutes, 44 earthquakes were recorded. Event magnitudes ranged from 0.1 to 1.8, with the majority between 0.6 and 1.4. Focal depths clustered predominantly around 8–11 km, although several events reached 15–16 km. The sequence exhibited typical swarm characteristics: no single dominant mainshock, clustered occurrence within a compact spatial volume, and rapid onset followed by gradual decay. The first two events (magnitudes 1.6 and 1.8) occurred within 15 minutes; subsequent activity remained low-magnitude and distributed across the three-day window, with the final event registering magnitude 1.4 at 13:08 on 25 May. Redlands occupies the San Bernardino Valley in southern California, a region shaped by the transform boundary between the Pacific and North American plates. The San Andreas Fault lies approximately 10 km to the north, while the San Jacinto Fault zone trends a few kilometers to the southwest. Both faults accommodate right-lateral strike-slip motion and have produced large historical earthquakes. The local basement consists of Mesozoic granitic and metamorphic rocks overlain by Quaternary alluvial and fluvial deposits that record ongoing uplift and sedimentation driven by faulting. Since 1 January 2000, nine swarms have been documented in the immediate vicinity, occurring in 2003 (1), 2005 (2), 2010 (1), 2013 (1), 2014 (1), 2015 (1), 2017 (1), and 2020 (1). These episodes illustrate the persistent, episodic nature of microseismicity along subsidiary structures associated with the major fault systems. The 2020 swarm’s depth distribution and modest magnitudes are consistent with shallow crustal adjustment along minor faults within the broader San Andreas–San Jacinto restraining-bend complex. Such swarms commonly reflect fluid migration or aseismic slip transients rather than progressive mainshock–aftershock sequences.

References

USGS Earthquake Catalog (ANSS Comprehensive Catalog)
Southern California Seismic Network (SCSN) annual reports
California Geological Survey, Regional Geologic Maps of the San Bernardino 30′×60′ quadrangle