Seismic Swarm S20200107.2: Analysis of Activity Near Ridgecrest, California

A seismic swarm designated S20200107.2 was recorded 11 km east-southeast of Ridgecrest, California. The sequence began at 05:17 on 7 January 2020 and concluded at 17:14 on 18 January 2020, spanning 275 hours and 56 minutes. During this period, 163 earthquakes were detected.

Analysis of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.4 to 2.2, with the majority below 1.0. Depths varied between 1 km and 11 km, clustering most frequently around 4–9 km. The initial event measured magnitude -0.3 at 4 km depth. Subsequent events on 7 January included a magnitude 2.2 quake at 6 km depth and several events near magnitude 1.0–1.8 at depths of 2–11 km. Activity continued at a steady rate through 8–11 January, featuring repeated magnitude 0.5–1.6 events at similar shallow depths. No events exceeded magnitude 2.2 in the examined subset, consistent with swarm characteristics of numerous small earthquakes rather than a dominant mainshock.

Ridgecrest lies within the Mojave Desert section of the Eastern California Shear Zone, a region of distributed right-lateral strike-slip faulting that accommodates a portion of Pacific–North American plate motion. The local geology features Quaternary alluvial fans and lacustrine deposits overlying Mesozoic basement rocks cut by numerous northwest-trending faults. The 2019 Ridgecrest earthquake sequence, which included magnitude 6.4 and 7.1 events, occurred on previously unmapped faults in this same area and produced extensive surface rupture.

Historical records indicate recurrent swarm activity in the region. Since 2000, 80 swarms have been documented, with notable yearly counts including 23 in 2019, 7 each in 2004 and 2010, and 6 each in 2006 and 2013. This 2020 swarm represents the sole recorded event of its type that year in the catalog.

Such swarms are common in the Walker Lane–Eastern California Shear Zone transition and are often attributed to fluid migration or aseismic slip on fault networks. Depths consistently under 11 km align with the brittle–ductile transition in this tectonically active crust.

References
USGS Earthquake Catalog
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center
USGS Professional Paper on the 2019 Ridgecrest Sequence