Seismic Swarm Analysis: La Verne, California – August 2018

A seismic swarm designated S20180829.1 occurred 4 km north of La Verne, California, from 02:33 on 29 August 2018 to 23:10 on 31 August 2018. In 68 hours and 37 minutes, 63 earthquakes were recorded. The sequence began with a magnitude 4.3 event at 5 km depth, followed by a magnitude 3.3 shock minutes later and numerous smaller events ranging from magnitude 0.4 to 2.4 at depths of 3–6 km. The swarm exhibited typical characteristics of clustered microseismicity without a dominant mainshock-aftershock pattern. Initial activity concentrated in the first hours, with the largest events occurring early on 29 August. Subsequent tremors declined in frequency and magnitude, concluding with a magnitude 1.3 event on 31 August. Depths remained shallow throughout, consistent with activity in the upper crust. La Verne lies within the eastern San Gabriel Valley at the northern margin of the Los Angeles Basin. This region forms part of the Transverse Ranges, where north-south compression from the Big Bend of the San Andreas Fault system drives uplift of the San Gabriel Mountains. Active faults nearby include strands of the Sierra Madre–Cucamonga fault zone, which accommodate reverse and left-lateral strike-slip motion. Historical seismicity in the area reflects ongoing strain accumulation along these structures. Since 1 January 2000, five prior swarms have been documented in the immediate vicinity: one each in 2002, 2005, 2008, 2012, and 2014. These recurrent clusters indicate that the local fault network periodically releases stress through distributed small-magnitude events rather than isolated large ruptures. Such behavior aligns with observations from other segments of the southern California fault system, where fluid migration or aseismic slip may trigger swarm sequences. The 2018 swarm did not produce reported damage or felt intensities exceeding moderate levels near the epicentral area. Its shallow focal depths and modest magnitudes limited potential ground shaking. Continued monitoring of the Sierra Madre–Cucamonga zone remains essential given the proximity to densely populated portions of Los Angeles County.

References

• United States Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• Southern California Seismic Network event data
• California Geological Survey fault activity maps