Yorba Linda Seismic Swarm of July–August 2008: Geological Context and Event Analysis

The Yorba Linda region in northern Orange County, California, lies within the Peninsular Ranges province at the northwestern margin of the Los Angeles Basin. This area experiences active transpressional deformation driven by the interaction between the San Andreas Fault system to the north and the Elsinore Fault Zone immediately to the east. The local geology is dominated by late Quaternary alluvial and fluvial sediments overlying Cretaceous granitic basement and Miocene sedimentary rocks. The 5 km NNE location of the swarm places it near the intersection of the Chino Fault and the Whittier Fault, both strands of the Elsinore system capable of producing moderate earthquakes.

Seismicity in this portion of the Elsinore Fault Zone typically occurs at depths of 10–18 km, consistent with the brittle–ductile transition in the regional crust. Historical records indicate that the area has hosted multiple moderate events, including the 2008 swarm that began at 18:42 UTC on 29 July 2008 and concluded at 18:46 UTC on 2 August 2008. During this 96-hour period, 128 earthquakes were recorded. The preceding swarm in the same locale occurred in 2002, marking the only other swarm episode documented since 1 January 2000.

Analysis of the initial 100 events reveals a classic swarm pattern without a single dominant mainshock-aftershock sequence. The largest event, magnitude 5.4, occurred at 15 km depth at the onset. Subsequent activity included a magnitude 3.8 event and a magnitude 3.6 event, both at approximately 16 km and 15 km depth, respectively. Magnitudes of the remaining events ranged predominantly between 0.5 and 2.7, with the majority falling below 2.0. Focal depths clustered tightly between 13 km and 18 km for the first three days, shallowing slightly toward the end of the sequence.

Temporal distribution shows the highest rate of occurrence within the first 12 hours, followed by a gradual decline. Events remained spatially concentrated within a few kilometers, indicating a compact source volume likely activated by fluid migration or aseismic slip along a subsidiary fault strand. No events exceeded magnitude 4.0 after the initial day, and the sequence exhibited typical swarm characteristics of overlapping occurrence times and lack of clear Omori-law decay.

This swarm underscores the persistent seismic hazard posed by the Elsinore Fault system to the greater Los Angeles metropolitan area. Although individual events were moderate, their proximity to densely populated communities highlights the importance of continuous monitoring and updated fault models for the northern Peninsular Ranges.

References

USGS Earthquake Catalog (queried for 2008 Yorba Linda sequence)
Southern California Seismic Network (SCSN) annual reports
California Geological Survey, Fault Activity Map of California (2010 update)