Seismic Swarm S20191014.1: Analysis of the October 2019 Event East of Ridgecrest, California

A seismic swarm designated S20191014.1 was recorded 17 km east of Ridgecrest, California, beginning at 10:17 on 13 October 2019 and concluding at 10:15 on 15 October 2019. Over 47 hours and 57 minutes, the sequence produced 36 earthquakes, with magnitudes ranging from 0.3 to 3.0 and focal depths between 2 km and 11 km. The events clustered tightly in time and space, characteristic of swarm behavior in which no single mainshock dominates.

The largest event reached magnitude 3.0 at 15:48 on 13 October at a depth of 4 km. A magnitude 2.7 earthquake occurred near the end of the sequence on 15 October at 10:15, also at 4 km depth. Other notable events included two magnitude 2.1 shocks on 13 October at depths of 2 km and 5 km. The majority of activity remained below magnitude 1.5, with 28 events between 0.3 and 1.2. Depths showed a bimodal distribution, with many events between 3–5 km and a secondary cluster near 7 km, suggesting activity across multiple fault strands or within a fractured volume.

This swarm occurred within the Eastern California Shear Zone, a region of distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific–North America plate motion. The Ridgecrest area lies near the intersection of the Garlock Fault and the Little Lake Fault zone. The 2019 Ridgecrest earthquake sequence of July, consisting of a magnitude 6.4 foreshock and magnitude 7.1 mainshock, ruptured previously unmapped faults and triggered widespread aftershock activity that continued for months. The October swarm represents continued post-mainshock adjustment in this structurally complex zone.

Historical records indicate that nine swarms have occurred in the region since 1 January 2000. One swarm took place in 2013, while eight occurred in 2019, underscoring elevated seismic productivity following the July mainshocks. Such swarms are common in the Mojave Desert, where fluid migration and stress transfer along intersecting faults can produce episodic clusters without a clear triggering mainshock.

The October 2019 swarm illustrates the ongoing seismic hazard in the Ridgecrest area. Although individual events were small, their occurrence within a recently activated fault network highlights the potential for larger triggered earthquakes. Continued monitoring remains essential for understanding fault interactions in this tectonically active portion of southern California.

References

USGS Earthquake Hazards Program, Ridgecrest Earthquake Sequence (2019).
California Geological Survey, Fault Activity Map of California.
Southern California Earthquake Data Center, Regional Seismicity Catalog.