Seismic Swarm S20190705.1: Analysis of Activity Near Ridgecrest, California

Seismic swarm S20190705.1 was recorded in the region 8 km southeast of Ridgecrest, California. The sequence began at 18:23 on 4 July 2019 and concluded at 04:20 on 18 September 2019, spanning 1809 hours and 56 minutes. A total of 8149 earthquakes were registered during this period. This activity occurred within the Eastern California Shear Zone, a tectonically active area characterized by distributed right-lateral strike-slip faulting that accommodates a portion of the Pacific-North American plate boundary motion.

The Ridgecrest area lies in the Mojave Desert, where northwest-trending faults interact with the east-west Garlock Fault. Geological studies indicate that the 2019 sequence involved rupture on previously unmapped conjugate faults, consistent with the region's history of complex, multifault earthquakes. Historical records show only two swarms in the area since 1 January 2000: one in 2013 with a single event and the 2019 swarm itself. These infrequent swarms highlight the episodic nature of seismicity in this portion of the shear zone.

Examination of the first 100 events reveals predominantly low-magnitude activity. Magnitudes ranged from 0.4 to 3.5, with the majority falling between 0.5 and 2.0. Depths were shallow, concentrated between 1 km and 12 km, and most events occurred at 4–8 km. The initial events on 4 July showed magnitudes of 1.4–2.1 at depths of 1–8 km. Activity continued through the night with similar characteristics. On 5 July, a notable 3.5 magnitude event occurred at 10:59 at a depth of 2 km, accompanied by several events exceeding magnitude 2.0, including a 2.6 at 11:51 and a 2.7 at 14:40. Depths remained shallow throughout, with no systematic deepening observed in this early phase.

This pattern of numerous small, shallow events is typical of swarm behavior driven by fluid migration or stress transfer along fault networks rather than a single mainshock-aftershock sequence. The events clustered in time, with bursts of activity separated by quieter intervals. Such characteristics align with documented swarm dynamics in the Eastern California Shear Zone.

The 2019 Ridgecrest sequence, of which this swarm formed a part, produced surface ruptures and provided valuable data on fault interactions in the region. Updated geological mapping confirms the involvement of multiple fault segments accommodating both strike-slip and normal components of motion.

References

USGS Earthquake Hazards Program reports on the 2019 Ridgecrest earthquakes.
California Geological Survey regional fault and seismicity summaries.
Peer-reviewed studies in Bulletin of the Seismological Society of America on Eastern California Shear Zone tectonics (2020–2023 updates).