Seismic Swarm S20170510.1: Analysis of the May 2017 Earthquake Sequence near Mountain Center, California

An earthquake swarm designated S20170510.1 was recorded 13 km southwest of Mountain Center, California, beginning at 14:36 on 9 May 2017 and concluding at 20:54 on 24 May 2017. Over 366 hours and 17 minutes, the sequence produced 129 events. This activity occurred within the tectonically complex Peninsular Ranges province of Southern California, a region characterized by active strike-slip faulting associated with the broader San Andreas transform boundary system. The San Jacinto Fault Zone, which transects the area, accommodates a significant portion of the relative motion between the Pacific and North American plates and has a well-documented history of both large earthquakes and episodic swarm activity.

The first 100 events of the swarm exhibited predominantly low magnitudes, with values ranging from 0.1 to 2.4. The largest event, magnitude 2.4, occurred at a depth of 4 km on 9 May 2017 at 16:22:10. Depths throughout the sequence clustered between 2 km and 15 km, with the majority concentrated between 4 km and 6 km, consistent with shallow crustal seismicity typical of the local fault network. Early activity on 9 May included multiple events near magnitude 1.0–1.7 at depths of 14–15 km, followed by a shift toward shallower foci as the swarm progressed. Subsequent days showed continued low-level energy release, punctuated by occasional events above magnitude 2.0, such as the pair of magnitude 2.1 and 2.0 quakes on 17 May at 9 km depth.

Historical records indicate that the region has experienced 31 swarms since 1 January 2000. Annual counts include single swarms in 2001, 2002, 2003, and 2009; two swarms each in 2005 and 2015; three swarms in 2010, 2012, and 2013; four swarms in 2011 and 2014; five swarms in 2016; and one swarm in 2017. This pattern underscores the recurrent nature of swarm-type seismicity in the vicinity, often linked to fluid migration or aseismic slip transients along subsidiary faults within the San Jacinto system.

The 2017 sequence remained entirely within the microseismic range, posing no significant hazard to nearby communities. Such swarms contribute valuable data for understanding stress transfer and fault interaction in one of California’s most active seismic corridors. Continued monitoring by regional networks supports ongoing assessment of background rates and potential precursory signals in this setting.

References

SeismoSight internal swarm classification S20170510.1 dataset.
USGS Earthquake Catalog for Southern California regional context.