Seismo-Watch

Special Earthquake Report No: 99-016

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CALIFORNIA / NEVADA BORDER REGION

5.6 Mb (NEIC), 5.7 Ml (UNRSL)

Mw 5.7 (NEIC, UC BERKELEY)

99/08/01
16:06:22
37.39N
117.07W
7 km
A

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An earthquake registering a preliminary magnitude of Mb 5.6 (NEIC), Ml 5.7 (UNRSL), Mw 5.7 (NEIC & UC Berkeley) was detected at 16:06:22 UTC (9:06 a.m. PDT), August 1, along the central California / Nevada Border region northeast of Death Valley National Park. The focal point was place at a depth of 7 km beneath the surface and the fault plane solutions by NEIC and UC Berkeley indicated normal-slip motion along a northeast trending plane, with a slight strike-slip component.
 
The quake was centered about 24 miles south-southeast of Goldfields and 34 miles northwest of Beatty, near Scotty's Junction (the intersection of Highway 95 and Highway 267). (See Map 1, Map 2, Map 3) It occurred in Nye County, Nevada just east of the Esmeralda / Nye County line, in Sarabatus Valley just east of Stonewall Pass. Highway 267 provides vehicle access from Highway 395 into northern Death Valley through Bonnie Claire Valley. For reference, Scotty's Castle, a popular tourist destination in Death Valley National Park, is at the base of the steep decent (the "Grapevine") from Bonnie Claire Valley into Death Valley. Scotty's Castle is about 26 miles southwest from the epicenter.
 
The earthquake shook the epicentral area quite hard and was felt as far away as Bishop, Bridgeport, Mammoth Lakes, Newberry Springs, Ridgecrest, California, California, to the west, Moapa and Las Vegas to the southeast, and Fallon, Eureka (Diamond Valley) and Austin to the north for a radius in excess of 165 miles.
 
Although there are no inhabitants in Sarabatus Valley and there have been no reports from Scotty's Castle at this time, the Amargosa Hotel at Death Valley Junction, located about 45 miles south of the epicenter, reported a swift, hard jolt followed by vigorous side-to-side shaking. Pictures were knocked from walls, items toppled from tables and shelves and furniture moved about the floor. Patrons at the hotel were frightened and ran from their rooms, some in their underwear. There have been no reports of any significant damage or injuries at this time. There were no reports of any sounds with the shaking.
 
At Las Vegas, people felt a slight swaying motion and some said water in their pools sloshed over the sides. Some of the tall hotels on the Vegas "Strip" swayed back and forth, causing some alarm to the patrons. For some travelers, it was their first "felt" earthquake. Several people in Eureka, approximately 160 miles northeast from the epicenter, reported feeling the ground move with long, rolling motions. Hanging plants swung in Ridgecrest and Newberry Springs, California, and residents in Mammoth Lakes, experienced veterans of many tremors, could tell this was a significant earthquake from outside their area.
 
According to the University of Nevada, Seismological Laboratory (UNRSL), in Reno, Nevada, the sequence began with a foreshock registering in the M3 range at 9:17 p.m. the night of July 31 which was followed by at least 12 foreshocks in the M2 range prior to the main M5.7 shock at 9:06 a.m., August 1. (See the UNRSL Press Release)
 
A vigorous aftershock sequence has followed, with tremor rates exceeding 20 eq/hr. More than a dozen M3 aftershocks were recorded by the UNRSL during the first 8 hours of the sequence, the largest of which registered about M3.7.Residents in Goldfield and attendants at Scotty's Castle have said they can feel many of the larger aftershocks. More often, they see ripples in their water glasses and do not feel the tremors.
 
UPDATE: An aftershock measuring M5.1 shook at 11:05 p.m., August 1, about 6 miles south of the main shock epicenter. Widely felt in the region and felt as far away as Eureka, Nevada. No damage reported.
 
Normal-slip fault motion, or down and away tectonic movement, is common in the Nevada Great Basin, thus giving rise to the stunning basin and range topography and the rich vegetation changes from the desert floors to the alpine ridgecrests. The northeast focal mechanism orientation is rather unusual, however.
 
The dominant fault in the region is the Death Valley - Furnace Creek fault zone, a prominent oblique-normal fault system which makes a significant component of the Walker Lane Fault Zone, a huge right-lateral megashear system which extends from about the Las Vega area to central Oregon. The maximum credible earthquake for the Death Valley-Furnace Creek fault zone in the upper M7 - lower M8 range - that is, if the whole fault zone ruptures at once.
 
Most of the major faults along the California / Nevada border follow the northwest Walker Lane topographic grain, however, there are a few structures which cut this geologic fabric, some of which occur in the vicinity of the Scotty's Junction earthquake. Two candidates for the likely fault that ruptured and caused the M5.7 earthquake would be the Bonnie Claire and Slate Ridge fault systems. Both are not well understood, yet today's earthquake was centered along a northeast projection of the Bonnie Claire fault which occupies a northeast diagonal valley between northern Death Valley, California, and Stonewall Valley, Nevada.
 
Some of the northeast trending valleys of the western Basin and Range are the result of rombohedral pull-apart basins produced by the Walker Lane megashear. The normal-slip (down and away to the southeast) would then be compatible with this motion and the formation of the northwest margin of the down-dropped Bonnie Claire basin. (click on image)
 
The aftershock pattern will be interesting and may shed some additional light on the tectonics surrounding the earthquake.
 
By the way, today's quake occurred close to a M5.5 temblor in 1910. More recently, a M4.1 earthquake hit west of Scotty's Castle on August 25, 1998.
 

Update: 8:00 a.m., August 3, 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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