Seismic Swarm S20251017.1 Near Loma Linda, California

A seismic swarm designated S20251017.1 occurred 3 km east of Loma Linda in Southern California. The sequence began at 13:07 on 16 October 2025 and concluded at 15:51 on 18 October 2025, lasting 50 hours and 44 minutes. During this period, 31 earthquakes were recorded, with magnitudes ranging from 0.6 to 2.0 and focal depths between 4 and 16 km.

The events clustered tightly in both space and time, characteristic of swarm activity rather than a mainshock-aftershock sequence. Early activity on 16 October included events at depths of 13–16 km, followed by a burst of shallower events around 6–8 km later that evening. Subsequent events on 17 and 18 October showed a return to deeper hypocenters. The largest event reached magnitude 2.0 at 23:40 on 16 October.

Loma Linda lies within the tectonically complex region of the Inland Empire, where the San Jacinto Fault Zone accommodates a significant portion of the Pacific–North American plate boundary motion. This right-lateral strike-slip system trends northwest–southeast and has produced multiple historical earthquakes exceeding magnitude 6. The swarm location aligns with the northern segment of the San Jacinto Fault, near its junction with the San Andreas Fault to the north. Crustal deformation in this area results from oblique convergence and distributed shear, producing frequent small-magnitude seismicity.

Since 2000, fourteen swarms have been documented in the immediate vicinity. Earlier episodes occurred in 2003, 2005 (two events), 2010, 2013, 2014, 2015, 2017, 2019 (three events), 2020 (two events), and 2025. These recurrent swarms reflect ongoing strain release along secondary faults and fracture networks within the broader San Jacinto system.

Such sequences typically arise from fluid migration or aseismic slip that triggers brittle failure on small fault patches. Depths recorded in S20251017.1 are consistent with the seismogenic zone in this part of the San Jacinto Fault, where brittle failure extends to approximately 15–18 km. No surface rupture or significant damage was reported, consistent with the modest magnitudes involved.

Continued monitoring remains essential given the proximity of the San Jacinto Fault to urban centers. The swarm provides additional data on the fine-scale structure and stress state of the fault zone.

References:
USGS Earthquake Catalog
Southern California Seismic Network
California Geological Survey Fault Database