Seismo-Watch

Special Earthquake Report No: 00-010

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TOPAZ LAKE AREA, CA/NV BORDER REGION

ML 4.8 (NCSN)

Mw 4.5 (UC Berkeley)

00/09/26
7:20:28 (12:20 a.m. local time)
38.691N
119.498W
10 km (6.2 miles)
C

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(Topaz Lake M4.8)

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TOPAZ LAKE, NEVADA, September 26, 2000 (Seismo-Watch) -- Preliminary data indicates that the largest of a series of earthquakes along the California-Nevada border southeast of Lake Tahoe near Topaz Lake registered ML 4.8, Mw 4.5.
 
The main earthquake occurred at 7:20 UTC or 12:20 a.m. PDT about 30 miles southeast of Lake Tahoe, 12 miles east of Markleeville and about 1 mile east of Topaz Lake beneath the Gray Hills in Antelope Valley. The focal point was placed at 10 km beneath the surface and the preliminary fault plane solution from the U.C. Berkeley suggested strike-slip motion along either a northwest (preferred) or northeast trending plane.
 
It was preceded by three foreshocks, a M1.7 a 11:56 p.m., September 25 and a pair of M2.9's at 2 and 9 minutes past midnight, September 26. It was quickly followed several aftershocks, including another temblor in the M4 range, a M4.2 at 12:27 a.m. Preliminary data showed the M4.2 quake had occurred about 3.5 miles southwest of Topaz Lake in Slinkard Valley, California, but reanalysis of the data indicated this not to be true and it was centered in the same location at the initial M4.8 jolt. Some of the aftershocks were also located in Slinkard Valley but this may be an artifact of the remote location of the activity and they probably are miss located. Perhaps further analysis will adjust the locations as well (See Seismo-Watch Regional Earthquakes for updated information). In all, 13 aftershocks were recorded, including three M3's and the M4.2 event.
 
The initial M4.8 earthquake was widely felt at Topaz Lake, Holbrook Junction, Smith Valley and Markleeville jarring many people from sleep. It was felt as far away as Lake Tahoe and Carson City, and we have a few unconfirmed reports from locations along the western side of the Sierra, including Paradise, Placerville, and Stockton. There have been no reports of items toppling from tables and shelves and there was no indications of injuries. There has been no word from the Highway Patrol or the Department of Transportation of fallen rocks on road ways. The Reno Gazette-Journal reported some additional information.
 
The main M4.8 shock was the first M4+ earthquake to occur in Nevada since a M4.2 struck Upper Fish Lake Valley (northeast of Bishop, CA and just inside the NV border) on March 14, 2000.
 
Seismo History:
A M6.0 earthquake occurred on September 12, 1994 about 11 miles northwest of Topaz Lake at Double Springs Flats. Robust aftershock sequence from that quake consisted of thousands of events, including several in Topaz Lake area as well with one as large as M4.7 on December 23, 1995 which caused a rock to topple off the hill and strike a truck near the California Inspection Station but in Nevada, causing it to crash and seriously injure the driver. It was the first earthquake related injury in Nevada in more than 30 years.
 

Sources: USGS, UCB, personal communications.

Update: 7:40 a.m., September 28, 2000.

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Copyright (c) Advanced Geologic Exploration 2000

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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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