Seismo-Watch

Special Earthquake Report No: 99-023
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HIGH DESERT, SOUTHERN CALIFORNIA

Ms 7.0 (SCEC), Ms 7.4 (NEIC)

Mw 7.0 (NEIC), Mw 7.0 (UCB), Mw 7.1 SCEC, Mw 7.1 HRV

99/10/16
09:46:44
34.60N
116.27W
6.0 km
A

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Notations:
An earthquake registering a preliminary magnitude of Ms 7.0 (SCEC), Mw 7.0 (NEIC, UCB), Mw 7.1 (Harvard, SCEC) was detected 09:46:44 UTC (2:46 a.m. PDT), October 16, in Southern California. The focal depth was fixed at a depth of 6.0 km for quick processing the fault plane solutions indicates strike-slip motion along a northwest trending plane.
 
The quake was centered 110 miles east of Los Angeles in a remote region of the High Desert of eastern Southern California about 32 miles northeast of Joshua Tree and 47 miles southeast of Barstow on the Twenty-Nine Palms Marine Corps Base, near Lavic Lake. (See Location Map) The quake struck on the eve of the 10th anniversary of the Loma Prieta (World Series) M7.0 earthquake in 1989.
 
The quake was preceded six minutes earlier by a M3.8 foreshock and has been followed by a robust aftershock sequence which has included a M5.3 at 2:51 a.m., a M5.8 at 5:57 a.m. and M5.0 at 10:38 a.m. Dozens of M3's and M4's have occurred in the aftershock zone.
 
The quake occurred east of the prominent Pisgah Fault and along the previously unnamed Lavic Lake Fault and has produced spectacular fault ruptures all of which occur within the U.S. Marine Corps Thousands Palms Base. Access on the Base is restricted and not advised due to active military operations using live rounds. In addition, the area is scattered with ordinances which may have not detonated. It is an extremely hazardous environment not advised for casual reconnaissance.
 
The USGS is reporting a fault rupture length of up to 40 km long. Surface fault ruptures show predominately oblique strike-slip movement with a complex rupture development, including en echelon steps, sympathetic and conjugate ruptures, normal and reverse-slip, mole-track fault features and pressure ridges and basins. Maximum displacements are on the order of 3-4 meters horizontal with up to 1.5 meter vertical. Pressure ridges may exceed 3 m of lift. Portions of the rupture zones are up to 200 m include major east and west facing graben development with internal deformation of the graben. The strike pattern of faulting is about N20°W, however, the rupture is sinuous and compounds the rupture pattern. More will be posted on the fault rupture later. See Fault Rupture Photos here
 
There has been regional sympathetic tectonic movement elsewhere in Southern California. In particular, a M4.8 struck the Johnson Valley Fault (Landers rupture zone) 1 mile Yucca Valley at 3:21 a.m. and a M5.3 occurred at 2:52 a.m. along the Homestead Fault (Landers rupture zone). There has also been some movement just east the San Andreas Fault in Coachella Valley near Indio where three tremors have measured in the mid to upper M3 range and along the southeastern margin of the Salton Sea at Obsidian Buttes where on of several events registered in the M4 range.
 
Damage Information:
Preliminary reports indicate the quake shook the region quite hard but there have been no word and serious injuries. Items topped from tables and shelves throughout the High Desert and the shaking woke people as far away as Los Angeles. It was felt as far away as San Diego, Phoenix and Las Vegas.
 
Commercial items in convenience and department stores in the High Desert were thrown from tables and shelves. Isles were littered with broken glass and debris. Some liqueur store owners have reported thousands of dollars of loss.
 
An Amtrak train passing by the epicenter near the town of Hector at the time of the earthquake derailed. It did not topple over but the surface waves bounced the train from the tracks. There were a few injuries on the train but none serious.
 
Several Interstate Highway 40 overpasses sustained some slight damage. One overpass that crossed Interstate Highway 40 near Ludlow sustained enough damage to be closed and one Interstate Highway 40 overpass over the railroad sheared a connection bolt and damaged the abutment.
 

Update: 10:30 p.m., October 19, 1999.


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Subtract 8 hours from Greenwich Mean Time to obtain PST or 4 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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