Seismic Swarm S20140705.1: Analysis of Activity Near Big Bear Lake, California

A seismic swarm designated S20140705.1 was recorded 10 km west-northwest of Big Bear Lake, California. The sequence began at 16:59 on 5 July 2014 and concluded at 23:12 on 14 July 2014, spanning 222 hours and 12 minutes. During this period, 164 earthquakes were registered.

The initial event reached a magnitude of 4.5 at a depth of 7 km. Subsequent activity included a magnitude 3.3 event one minute later at 8 km depth. The first 100 events displayed predominantly low magnitudes, with the majority falling between 0.4 and 2.0. Depths ranged consistently from 6 to 10 km, indicating a shallow crustal source. Notable secondary events included magnitudes of 2.4, 2.0, and 3.0, all occurring within the first three days. Frequency peaked immediately after the mainshock and declined steadily, consistent with swarm behavior rather than a classic mainshock-aftershock sequence.

Analysis of the listed events reveals a median magnitude near 0.9 and an average depth of approximately 7.5 km. Over 70 percent of the initial 100 events registered below magnitude 1.5, underscoring the swarm’s microseismic character. No events exceeded magnitude 4.5 during the recorded interval.

The Big Bear Lake region lies within the San Bernardino Mountains of the Transverse Ranges, a zone of active compression and strike-slip faulting driven by the Pacific-North American plate boundary. The area is bounded by segments of the San Andreas Fault system to the south and the North Frontal Fault system to the north. Historical seismicity includes the 1992 Big Bear earthquake (M6.5), which occurred on a previously unrecognized fault and produced widespread shaking across Southern California.

Since 2000, four prior swarms have been documented in the immediate vicinity: one each in 2001, 2003, 2005, and 2010. These episodes demonstrate recurrent, clustered seismicity without progression to larger mainshocks, suggesting localized stress release on minor faults or fluid-influenced fracture networks within the crystalline basement rocks.

This swarm reinforces the region’s persistent low-to-moderate seismic hazard. Continuous monitoring by regional networks remains essential for distinguishing swarm activity from potential foreshock sequences.

References

• SeismoSight internal swarm classification records
• USGS Earthquake Catalog (event data cross-verification for regional context)