SeismoSight Analysis: Earthquake Swarm S20151230.1 Near Devore, California

The earthquake swarm designated S20151230.1 occurred 5 km south of Devore in San Bernardino County, Southern California. It began at 06:52 on 29 December 2015 and concluded at 21:58 on 3 January 2016, spanning 135 hours and 5 minutes. During this interval, 111 earthquakes were recorded. This activity took place in a tectonically active zone at the intersection of the San Andreas Fault and the San Jacinto Fault within the Transverse Ranges.

Southern California lies along the boundary between the Pacific and North American plates, where right-lateral strike-slip motion produces frequent seismic events. The Devore area features a network of faults accommodating regional strain, with the Cajon Pass serving as a critical segment. Depths of the recorded events ranged primarily between 2 km and 10 km, consistent with shallow crustal seismicity typical of this transform boundary setting. The largest event reached magnitude 4.4 at a depth of 6 km on 30 December 2015 at 01:48:57, followed by an immediate sequence of aftershocks including magnitudes 3.7, 3.0, and 2.8 within the first hours.

Analysis of the first 100 events reveals a classic swarm pattern marked by a rapid onset after the initial larger shock, with the majority of subsequent events below magnitude 2.0. Magnitudes clustered between 0.4 and 2.3 after the peak, and depths remained stable in the 4–8 km range for most occurrences. Temporal distribution showed highest frequency in the initial 24 hours, gradually tapering over the following days. This distribution aligns with fluid migration or stress triggering mechanisms often observed in fault step-over regions.

Historical records since 2000 indicate four prior swarms in the immediate vicinity, occurring in 2003, 2005, 2013, and 2014. These episodes underscore the recurrent nature of clustered seismicity here, likely driven by the geometric complexity of intersecting fault strands that allow episodic aseismic slip or pore-pressure changes to influence multiple segments simultaneously.

The broader geological context includes Quaternary alluvial deposits overlying metamorphic and granitic basement rocks of the San Gabriel Mountains. Ongoing convergence and lateral shear in the Transverse Ranges contribute to elevated seismic hazard, with the region experiencing both mainshock-aftershock sequences and swarm-type activity. Updated monitoring by regional networks confirms continued low-level background seismicity consistent with long-term strain accumulation along the southern San Andreas system.

This swarm provides valuable data on fault interaction in a high-risk corridor near major transportation routes and urban centers. Continued observation supports improved models of short-term seismic clustering in Southern California.

References

USGS Earthquake Hazards Program
California Geological Survey Fault Activity Map
Southern California Earthquake Data Center Catalog