Seismic Swarm S20190706.3: Analysis of Activity East of Ridgecrest, California

The seismic swarm designated S20190706.3 occurred 8 km east of Ridgecrest, California, within the Eastern California Shear Zone. This region lies at the boundary between the Pacific and North American plates, where distributed right-lateral strike-slip faulting accommodates a portion of the relative plate motion. The swarm initiated at 22:59 on 5 July 2019 and concluded at 00:40 on 13 July 2019, spanning 169 hours and 41 minutes during which 190 earthquakes were recorded.

Analysis of the first 100 events reveals predominantly shallow focal depths between 0 km and 20 km, with the majority clustered between 4 km and 10 km. Magnitudes ranged from 0.6 to 3.8, indicating a typical swarm distribution dominated by smaller events. Notable larger events included a magnitude 3.8 earthquake at 07:54 on 6 July at 8 km depth and several magnitude 3.2–3.6 shocks distributed across the following days. Depths showed no consistent migration pattern, though many events occurred at or near 0–2 km, consistent with activity on near-surface fault segments in this arid, fractured terrain.

The Ridgecrest area has experienced recurrent seismic swarms. Since 1 January 2000, 55 swarms have been documented in the vicinity, with annual counts varying from one to seven. Peak activity years include 2004 and 2010, each recording seven swarms, followed by 2006 with six. The 2019 sequence contributed four swarms, aligning with the long-term average frequency.

Geologically, the swarm location sits amid Quaternary alluvial fans and faulted bedrock of the Mojave Desert. The Eastern California Shear Zone comprises numerous discontinuous faults that have produced historical surface-rupturing events. The 2019 Ridgecrest sequence, of which this swarm formed a component, highlighted the potential for complex, multi-fault rupture in previously unmapped structures. Regional strain rates derived from geodetic measurements indicate ongoing deformation at several millimeters per year, supporting the observed microseismicity.

This swarm underscores the importance of continuous monitoring in the shear zone. Event statistics demonstrate the value of dense seismic networks in capturing the full spectrum of activity that may precede or accompany larger ruptures.

References

• United States Geological Survey Earthquake Hazards Program, Ridgecrest Earthquake Sequence reports (2019)
• California Geological Survey, Fault Activity Map of California
• Southern California Earthquake Data Center, regional seismicity catalogs since 2000