Seismic Swarm S20200109.1 Near Aguanga, California: Event Analysis and Regional Context

Seismic swarm S20200109.1 occurred 11 km northeast of Aguanga in Riverside County, California. The sequence began at 07:19 on 8 January 2020 and concluded at 16:31 on 19 January 2020, spanning 273 hours and registering 159 earthquakes.

Analysis of the first 100 recorded events reveals predominantly low-magnitude activity. Magnitudes ranged from -0.2 to 2.4, with the majority below 1.0. Depths concentrated between 2 km and 6 km, though several events reached 12–15 km. The largest event, magnitude 2.4, occurred early in the sequence on 8 January at 14:40 at a depth of 1 km. Subsequent peaks included magnitudes of 2.1 and 1.9 on the same day and 9 January. Activity showed clustering on 8–10 January, followed by a gradual decline through 15 January, with events becoming sparser and shallower on average.

This swarm fits within a documented pattern of seismic activity in the region. Since 1 January 2000, 41 swarms have occurred in the same area. Annual counts remained low through the mid-2000s before rising sharply after 2014, reaching seven swarms in 2018 alone. The 2016–2018 period accounted for 18 swarms, indicating elevated swarm frequency in recent years.

The Aguanga area lies within the Peninsular Ranges province of southern California, part of the broader San Andreas Fault system. Local faulting includes strands of the Elsinore Fault zone and subsidiary structures that accommodate right-lateral shear between the Pacific and North American plates. Shallow crustal depths typical of the recorded events align with the brittle upper crust in this transform boundary setting. Historical seismicity shows that swarms in this locale often result from transient stress changes or fluid migration along pre-existing fractures rather than mainshock-aftershock sequences driven by a single large rupture.

The provided swarm parameters and event list constitute the primary data source for this report. No external verification of individual event times, magnitudes, or depths was performed.

References

• SeismoSight internal swarm classification records (S20200109.1 parameters and event list)
• Regional tectonic framework derived from standard geological descriptions of the Peninsular Ranges and Elsinore Fault zone