The 2002 Yorba Linda Earthquake Swarm: Geological Context and Event Analysis

The Yorba Linda region in Orange County, California, lies within the tectonically active Los Angeles Basin, where north-south compression arises from the Big Bend in the San Andreas Fault system. This setting produces reverse and strike-slip faulting along structures such as the Whittier Fault and the nearby Chino and Puente Hills faults. The basin’s sedimentary fill, overlying crystalline basement rocks, amplifies seismic shaking and hosts frequent small-magnitude events.

On 3 September 2002 at 01:50 UTC, a seismic swarm began 6 km northeast of Yorba Linda. The sequence continued until 20:44 on 4 September 2002, spanning 42 hours and 53 minutes and producing 39 earthquakes. The largest event, magnitude 4.7 at 07:08 on 3 September, occurred at a depth of 9 km and was followed within minutes by several events of magnitude 2.0–2.9 at depths between 10 and 12 km. Subsequent activity showed a gradual decline in both rate and magnitude, with the final recorded event of magnitude 1.4 at 5 km depth closing the swarm.

Depths throughout the sequence ranged from 4 km to 15 km, clustering most densely between 9 km and 13 km. This distribution is consistent with the regional seismogenic zone, where brittle failure occurs within the upper crust above the ductile transition. The swarm exhibited classic characteristics of fluid-assisted or aseismic-slip-triggered sequences, with no single dominant mainshock-aftershock pattern; instead, multiple events of comparable size occurred within the first few hours.

Historical records indicate similar swarm-like episodes near the Whittier Fault in the late twentieth century, underscoring the area’s persistent low-to-moderate seismic hazard. Updated assessments from regional monitoring networks confirm that the Puente Hills and Whittier systems remain capable of producing damaging earthquakes, although the 2002 swarm released only modest energy.

The temporal evolution—rapid onset, peak activity within six hours, and steady decay over the following day—provides a clear example of swarm behavior in a compressional basin setting. Continued monitoring of microseismicity in this zone aids in refining fault models and improving short-term hazard estimates for the densely populated Los Angeles metropolitan region.

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