Earthquake Alert Bulletin No: 98-259

Preliminary data indicates a significant earthquake has occurred:

Regional Location:

Preliminary Magnitude:

Moment Magnitude:

Greenwich Mean Date:
Greenwich Mean Time:
Focal Depth (km):
Analysis Quality (A-D):


4.9 ML (SCSN)

4.3 Mw (Caltech)

01:08:40 (5:08 pm PST)
4.3 km


An earthquake registering a preliminary magnitude of ML 4.9 (SCSN), Mw 4.3 (Caltech), Mw 4.5 (Berkeley) occurred at 01:08:40 UTC (5:08 p.m. PST, October 26, in Southern California. The focal point, or area of first rupture, was placed at a depth of 4.3 km beneath the surface. The quake was centered about 4 miles north of Big Bear City along the northern flank of the San Bernardino Mountains in Cushenberry Canyon at Whisky Springs. It was probably centered on the southern extension of the Helendale fault, but is too early to make a positive confirmation The focal mechanisms by Caltech and Berkeley indicated strike-slip motion along a northwest trending fault plane which agrees with the orientation of the Helendale fault. The earthquake will probably be referred to as the Whisky Springs M4.9 earthquake.

Numerous aftershocks have been recorded, including a dozen M2's a pair of M3's and one as large as ML 4.2 (SCSN) at 7:40 a.m. Tuesday morning. That aftershock was also widely felt but did not cause any damage. The earthquake sequence is not related to the Big Bear M6.6 earthquake which followed the Landers M7.3 earthquake in 1992. The sequence location is much furthern north and the Big Bear aftershock sequence died out a couple of years ago. These were the third fourth M4 quakes to shake the San Bernardino region in as many months. The other two occurred south of Big Bear near Mountain Home Village. Also a M4.5 jolt occurred in the San Bernardino Mountains on March 11 earlier thhis year.

Seismo-Watch received felt earthquake reports from several people in San Bernardino and Riverside Counties, and as far away as Santa Monica on the coast. People reported their hanging plants swung, windows rattled and items on counters jiggled. There was one report received by the USGS of of items toppling from tables or shelves but there were no details. Report your felt earthquake observations here.

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Subtract 8 hours from Greenwich Mean Time to obtain PST or 5 hours for EDT
Location Quality: A (good), B (fair), C (poor), D (bad)
Magnitude: Ml (local or Richter magnitude), Lg (mblg), Md (duration), Mb (body wave), Ms (surface wave), Mw (moment)

Standard Sources Include:
U.S. Geological Survey, National Earthquake Information Center, Golden, CO (NEIC)
Harvard Geophysical Observatory, Harvard University, Cambridge, MA (HRV)
U.S. Geological Survey, Northern California Seismic Network (NCSN)
University of California, Berkeley, Seismological Laboratory (UCBSL)
California Institute of Technology, Pasadena, California (Caltech)
various wire reports and/or personal communications

All data are preliminary and subject to change.
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