Seismic Swarm S20010210.1: Analysis of February 2001 Activity Near Big Bear Lake, California

A seismic swarm designated S20010210.1 occurred in the vicinity of Big Bear Lake, California, beginning at 07:41 on 10 February 2001 and concluding at 23:06 on 16 February 2001. Over 159 hours and 24 minutes, the sequence produced 116 earthquakes. The epicentral area lies approximately 6 km NNW of Big Bear Lake in the San Bernardino Mountains. This region forms part of the Transverse Ranges province and lies within the broad zone of deformation associated with the Pacific-North American plate boundary. The San Andreas Fault system exerts primary control on regional tectonics, with additional influence from the North Frontal Thrust Fault and subsidiary strike-slip structures. Historical seismicity includes the 28 June 1992 magnitude 6.5 Big Bear earthquake, whose aftershock zone extended into the same area. The 2001 swarm occurred within a well-documented zone of distributed crustal strain where moderate-magnitude events and swarms recur on timescales of years to decades. Analysis of the first 100 recorded events reveals a compact spatial and temporal distribution. Magnitudes ranged from 0.7 to 4.6, with the largest shock (magnitude 4.6) occurring at 21:05 on 10 February at a focal depth of 7 km. Subsequent notable events reached magnitudes 4.2, 3.5, 3.4, 3.2, 3.1, and 3.0. Depths were predominantly shallow, clustering between 3 km and 9 km, consistent with brittle failure within the upper crust. A secondary cluster of events between 10 km and 13 km suggests limited activity on slightly deeper structures. The temporal pattern shows an initial intense phase on 10 February, followed by a gradual decline punctuated by occasional larger events through 14 February. The sequence lacked a single dominant mainshock-aftershock signature and instead displayed the characteristic swarm behavior of numerous events of similar size occurring without clear migration. Most events were below magnitude 2.5, indicating efficient release of strain through small ruptures on a network of closely spaced faults. Such swarms are common in the Big Bear Lake area and are generally attributed to fluid migration or aseismic slip triggering brittle failure on favorably oriented planes. The 2001 sequence did not produce surface rupture or significant damage, consistent with its moderate maximum magnitude and shallow but contained depths.

References

SeismoSight internal swarm classification S20010210.1
USGS Earthquake Catalog (historical regional events)
California Geological Survey, Regional Geologic Map Series