Seismic Swarm S20190710.1 Near California City, California

Seismic swarm S20190710.1 occurred 25 km north-northeast of California City in the Mojave Desert region of California. The sequence began at 22:11 UTC on 9 July 2019 and concluded at 18:25 UTC on 16 July 2019, spanning 164 hours and 14 minutes. During this period, 98 earthquakes were recorded, with magnitudes ranging from 0.6 to 3.3 and focal depths predominantly between 0 and 11 km.

The Mojave Desert lies within a tectonically active zone influenced by the Pacific-North American plate boundary. Regional deformation is accommodated by a network of strike-slip and normal faults, including the Garlock Fault to the north and segments of the Eastern California Shear Zone. Shallow crustal seismicity is common due to the brittle nature of the upper crust in this arid, extensional environment. Historical records indicate recurrent low-to-moderate magnitude events, often occurring in clusters rather than isolated mainshock-aftershock sequences.

This swarm fits the pattern of episodic seismic activity documented in the area since 2000. Only two prior swarms have been identified: one in 2009 comprising a single event cluster and another in 2016 with similar characteristics. Such swarms typically involve numerous small events without a dominant mainshock, reflecting fluid migration or aseismic slip processes along pre-existing fault planes.

Event distribution showed peak activity on 10 July, with multiple events exceeding magnitude 2.0, including a 3.1 at 19:15 UTC. Subsequent days featured sustained but diminishing rates, with the largest event of the sequence—a magnitude 3.3—occurring on 15 July at 06:39 UTC. Depths remained shallow throughout, consistent with the regional seismogenic zone extending to approximately 15 km.

Analysis of temporal patterns reveals an initial burst followed by gradual decay, characteristic of swarm behavior driven by localized stress perturbations rather than a single rupture. Magnitudes clustered below 2.0 for the majority of events, indicating limited energy release compared with the 2019 Ridgecrest sequence located farther north.

Continued monitoring in this portion of the Mojave Desert remains essential given its proximity to major fault systems and population centers such as California City and Ridgecrest. Improved detection networks have enhanced resolution of these low-magnitude sequences, supporting refined hazard assessments for the Eastern California Shear Zone.

References

SeismoSight internal swarm catalog S20190710.1
USGS Earthquake Catalog (regional Mojave Desert events)
California Geological Survey fault database