The location of the main shock, as given by the California Institute of Technology - U. S. Geological Survey seismic network is 34.60N 116.27W , and at a depth of 6.0 km. Of course, an event of this size is expected to rupture the fault from the surface to a depth of 10 to 15 km, and along a horizontal distance of about 30-50 km. Thus this hypocenter represents only the initial point of energy release in an event that will have deformed the earth over a large region.
The location as given above is on the Twentynine Palms Marine Corps Base. The nearest major fault is the Pisgah fault, which is a northwest-striking strike slip fault. The preliminary location is about 10 km east of that fault, however, so it might turn out that a smaller nearby fault is involved instead. I expect those uncertainties to be resolved quickly (10:00 AM, Oct 16). The region has a large number of subparallel strike-slip faults. The focal mechanism is strike-slip motion, consistent with the mechanism expected on the Pisgah fault or on the nearby related faults. The initial aftershock locations are distributed in a northwest trend similar to what one would expect for an earthquake in this region.
The earthquake was felt strongly in Las Vegas, where typical reports describe 20-30 seconds of motion (total range 10-90), hanging objects swinging noticably, some unstable objects overturned, creaking of walls or other noises caused by the building. The motions were described as rolling, awakening most people from the homes of people reporting. Our reports indicate that many people responded wisely by moving to a doorway or outdoors; they generally do not report difficulty standing, but some reported some dizziness and that walking was a little uneasy. Our most northern felt report so far comes from Carson City, where a lady was awakened and some chimes within her house were rung.
The faults in the eastern Mojave Desert resemble faults in Nevada in the sense that there are a large number of faults there, each of which has a very low probability of a significant earthquake. However, the net effect of the large number of faults in the region is that the probability of an earthquake someplace in the region becomes important.
The earthquake may be compared with the somewhat larger Landers, California, earthquake on 28 June, 1992 (Mw=7.3). That earthquake also had a strike-slip mechanism on a northwest-oriented fault. Both of those earthquakes illuminate the mechanism by which strain is transferred from the San Andreas system to the faults on the east side of the Sierra Nevada mountains. The net effect of this is that about 20% of the total motions between the Pacific plate and the North America plate occur on the east side of the Sierras, and contribute to the Nevada earthquake hazard.
The Landers earthquake triggered a significant number of tiny earthquakes along the eastern Sierra Nevada mountains. Our preliminary check of the records suggests that the Hector Mine earthquake has not had as strong an effect, although there appears to be an increase in tiny earthquakes near Mammoth Lakes. For more on this earthquake see http://www.seismo.unr.edu/htdocs/WGB/Recent.old/HectorMine.
John Anderson, Director, Nevada Seismological Laboratory
October 16, 1999
10:55 AM
The data reported here are preliminary. For more information, contact the Seismological Laboratory, (775) 784-4975 or on the Internet at http://www.seismo.unr.edu. The free pamphlet "Earthquakes in Nevada and How to Survive Them, is available on request.