A M7.1 earthquake occurred at 2:46 a.m. local time on 10/16/1999. The event was located in a remote, sparsely-populated part of the Mojave desert, approximately 47 miles east-southeast of Barstow and 32 miles north of Joshua Tree. The initial magnitude estimate of 7.0 was upgraded to 7.1 on October 18, 1999, based on in-depth analysis of "teleseismic" data recorded worldwide. Like music, earthquake waves include both "high tones" and "low tones"--the latter, which are recorded by sensitive seismic instruments around the globe--are crucial in constraining the magnitude of large events, but are not analyzed quite as quickly as data from the immediate southern California region.
The Hector Mine earthquake is not considered an aftershock of the M7.3 Landers earthquake of 1992, although subsequent analysis will explore the relationship between these two events.
On average, an earthquake of M7.0 is expected to generate approximately seven aftershocks of M5 or larger within the first week. Three M>5 events occurred on the first day of the sequence: a M5.3 event at 2:49 local time, a M5.8 event at 5:57 a.m., and a M5.0 at 10:38 a.m. 25 aftershocks of M4 or larger occurred within the first 36 hours of the sequence. The rate of aftershocks is very close to the average expected for a mainshock of this magnitude.
The earthquake occurred on the Lavic Lake fault, one of a series of north-northwest trending faults through the eastern Mojave shear zone. Geologists from the U.S. Geological Survey and Southern California Earthquake Center were able to fly over the rupture (which is within the Twenty Nine Palms Marine Base) on the afternoon of October 16 and documented a 40-km long surface rupture. The aerial photos show an apparent maximum offset of 3.8 - 4.7 meters. These preliminary estimates for both fault length and slip are consistent with expectations for a M7.0 earthquake, although the slip is perhaps somewhat larger than average.
The Lavic Lake fault had been previously mapped in the Bullion Mountains by Thomas Dibblee, Jr., of the USGS, but the fault was not named. It is one of a series of closely spaced, northwest-trending, right-lateral strike slip faults that traverse this portion of the Mojave Desert. Together, these faults within what is termed the Mojave Shear Zone serve to relieve a small portion of the stresses that build up along the boundary between the Pacific and North American tectonic plates.
Previous evaluations of the Lavic Lake fault by the California Division of Mines and Geology have shown that the fault had not produced a large earthquake within the last 10,000 years. As a result, the fault was not considered "active" according to the regulatory criterion set forth in the Alquist-Priolo Earthquake Fault Zoning Act of 1972. This statute requires extensive follow-on fault studies and special building setbacks before development can proceed along "active" faults. However, movement along faults within the Mojave Shear Zone is typically quite slow, and the individual faults produce earthquakes only infrequently. In this type of setting, then, the 10,000 year criterion for fault activity, established nearly 30 years ago, may not be particularly appropriate.
An interesting aspect of the Hector Mine earthquake is that it occurred only 7 years after the 1992 Landers and Joshua Tree earthquakes, which occurred on similar faults within the Mojave Shear Zone. This apparent clustering of earthquakes may be purely coincidental, but scientists are conducting research to see if this kind of earthquake behavior is typical of this region. That is, perhaps many of the Mojave Shear Zone faults produce earthquakes within a short time interval (perhaps spanning several hundred or more years), followed by several thousands of years of quiescence. The Hector Mine earthquake will provide important data that will enable scientists to answer this and other questions about faulting and earthquake behavior.
Geologists are working to map the surface rupture in detail. Scientists with Caltech and the US Geological Survey are continuing to analyze the abundant seismic data recorded from the Hector Mine Earthquake and its aftershocks. In addition to the classic aftershock sequence, the earthquake has apparently triggered small earthquakes as far south as the California-Mexico border. Scientists are monitoring these events carefully, as some of them have occurred close to the southern end of the San Andreas fault.
The Southern California Seismic Network is in the process of a major upgrade involving installation of state-of-the-art instrumentation to provide far more detailed information about shaking than has been available in the past. Real-time results from the new Trinet network were able to produce ShakeMaps showing the distribution of shaking over Southern California. In addition, over 23,000 people have electronically filled out a questionnaire for the Did You Feel It? Community Internet Intensity Map (CIIM).
The SCIGN Project is also in the process of installing a state-of-the-art integrated GPS monitoring system in southern California. The SCIGN results for the Hector Mine earthquake show the co-seismic displacement associated with the earthquake.