Seismo-Watch

Special Earthquake Report No: 99-021

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N. SAN BERNARDINO MTNS., SOUTHERN CALIFORNIA

4.2 Ml (SCSN)

n/a ( -- )

99/09/20
07:02:49
34.32N
116.85W
5.6 km
A

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An earthquake registering a preliminary magnitude of ML 4.2 (SCSN) was detected 07:02:49 UTC (12:02 a.m. PDT), September 20, in the interior of Southern California. The focal depth was placed at a depth of 5.6 km and we do not have a fault plane solution for this event as of yet.
 
The quake was centered on the north side of the San Bernardino Mountains about 4 miles north of Big Bear City in Cushenberry Canyon, at Whisky Springs. The quake occurred in essentially the same location as the Whiskey Springs M4.8 earthquake on October 26, 1998.
 
The main jolt was widely felt in the Inland Empire, waking several people from sleep. There have been no reports of items toppling from tables or shelves at this time.
 
Whereas that earthquake produced a robust aftershock sequences which included a couple of M4 aftershocks, today's temblor was followed by only a couple of aftershocks, the largest of which measured M2.6.

For those keeping track, it was the eighth earthquake in Southern California this year. Including the activity in northern Baja, Mexico, however, the tally doubles to 16 events. Since further analysis of the May 14 Joshua Tree earthquake determined a magnitude of M4.9 rather than the preliminary M5.0 reported earlier, it has been since the Coso Range double M5.2 and M5.0 earthquake series in March 1998 since the last M5 earthquake in Southern California (and that includes the region of northern Baja), making a gap between M5+ events of 19 months. A gap this long for M5+ events has not been seen in Southern California in 20 years and since the 4.5 year drought between December 7, 1976 (M5.5) and April 26, 1981 (M5.7).


Update: 8:00 a.m., September 20, 1999.

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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)
Southern California Seismic Network (USGS & Caltech), Pasadena, California (SCSN)
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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