Seismic Swarm S20161230.1: Analysis of Activity Near Anza, California

A seismic swarm designated S20161230.1 occurred 13 km east-southeast of Anza, California, from 13:01 on 29 December 2016 to 20:25 on 2 January 2017. Over 103 hours and 23 minutes, 48 earthquakes were recorded. The events clustered tightly in time and space, with magnitudes ranging from -0.2 to 2.8 and focal depths primarily between 7 and 13 km.

The sequence began with low-magnitude events on 29 December, including a 1.6 event at 20:33. Activity intensified on 30 December, featuring the swarm’s largest earthquake, a magnitude 2.8 at 05:33, followed by numerous aftershocks of 0.2–1.3. Smaller events continued through 31 December and into 1–2 January, tapering off by the final recorded quake at 20:25 on 2 January. Depths remained consistent in the upper to mid-crust, consistent with typical shallow faulting in the region.

This swarm fits a well-documented pattern of clustered seismicity in the Anza area. Since 1 January 2000, 35 swarms have occurred there, with notable yearly counts including five each in 2010 and 2016, four each in 2011 and 2013, and three each in 2012 and 2014. Such episodes reflect episodic strain release along active fault segments rather than isolated mainshock-aftershock sequences.

The Anza region lies within the San Jacinto Fault Zone, a major right-lateral strike-slip system that forms part of the boundary between the Pacific and North American plates. The San Jacinto Fault accommodates a significant portion of the plate-boundary slip rate, estimated at 10–15 mm per year. The local geology features a complex network of faults cutting through Mesozoic crystalline basement rocks and overlying sedimentary units of the Peninsular Ranges. The Anza Gap, a 20 km-long section of the fault immediately northwest of the swarm location, has not produced a large surface-rupturing earthquake in historical time, leading to its identification as a potential seismic gap.

Seismicity in this zone is driven by the regional tectonic regime of northwest-directed dextral shear. Small-magnitude swarms like S20161230.1 commonly occur at depths of 5–15 km, where brittle failure predominates. These events provide data on fault mechanics and stress accumulation, contributing to ongoing hazard assessments for the greater Los Angeles–San Diego corridor.

References:
USGS Earthquake Catalog
Southern California Seismic Network annual reports
California Geological Survey fault database