Earthquake Swarm S20100607.1 Near Big Bear City, California

An earthquake swarm designated S20100607.1 was recorded beginning at 12:50 on 7 June 2010 and concluding at 00:27 on 11 June 2010. The events were centered 6 km north of Big Bear City, California. Over 83 hours and 37 minutes, a total of 108 earthquakes were registered.

Analysis of the first 100 events reveals predominantly low-magnitude activity. The largest event reached magnitude 3.3, occurring at 20:32:56 on 7 June 2010. Other notable magnitudes included 3.2, 2.7, 2.6 (twice), and 2.5 (four times). Depths were generally shallow, with most events between 0 and 5 km; several registered at negative depths, consistent with typical cataloging variations for very shallow sources. Activity was most intense during the evening of 7 June and midday on 8 June, after which the rate declined steadily through 10 June.

This swarm fits within the broader seismic character of the region. Big Bear City lies in the San Bernardino Mountains of the Transverse Ranges, where the San Andreas Fault system interacts with secondary structures such as the North Frontal Fault Zone. The area experiences frequent small-magnitude earthquakes due to right-lateral strike-slip motion along the San Andreas and distributed deformation across adjacent faults. Historical records indicate two prior swarms in the immediate vicinity since 2000: one event in 2001 and one in 2003.

The June 2010 swarm produced no reported damage or injuries, consistent with its maximum magnitude remaining below 4.0. Such sequences are common in the region and typically reflect fluid migration or minor stress adjustments rather than foreshock activity preceding a larger mainshock.

References

• U.S. Geological Survey Earthquake Catalog (earthquake.usgs.gov)
• California Geological Survey, Regional Fault Maps
• SeismoSight internal swarm classification records