|Surface Rupture from the M7.1 Hector Mine Earthquake|
(Photographic base courtesy of the Marine Corps Air Ground Combat Center, Twentynine Palms)
Above is a diagrammatic representation of surface faulting produced by the M7.1 Hector Mine earthquake of October, 16, 1999. This figure was derived from mapping by more than 40 geologists from the U.S. Geological Survey, Southern California Earthquake Center, and the California Division of Mines and Geology. The principal rupture is shown in yellow. It extends for more than 41 km, although the portion of the rupture with observed displacement of >1 cm (shown here) is barely more than 35 km long.
The rupture involves portions of two previously mapped fault zones - the Bullion fault, and an unnamed, more northerly trending fault that is now informally referred to as the Lavic Lake fault. This pattern of rupture along more than one named fault was also observed from the 1992 Landers earthquake. The closely spaced nature of the surface faults in this area (other major mapped faults in the region are shown with dashed green lines) suggests that they are interrelated and connected to a fewer number of "master" faults at depth.
Much of the fault zone that produced the Hector Mine earthquake had been buried by relatively young stream deposits and the fault scarps in bedrock have a subdued morphology. Thus it appears that these faults have not experienced significant offset for perhaps 10,000 years or more. Planned future investigations will refine the age of the last event on these faults. The portion of the Lavic Lake fault that ruptured between the northern end of the Bullion Mountains and Lavic Lake had not previously been mapped. However, our current field investigations have identified ancient, subdued fault scarps along the 1999 rupture zone in this area. Thus, it appears that the entire Lavic Lake fault that was involved in the 1999 event had ruptured before.
The greatest fault displacement, about 5.2 m of right-lateral strike slip (largest value measured as of 10/23/1999), occurs on the Lavic Lake fault, near the epicenter in the Bullion Mountains (see Preliminary Fault-Slip Distribution from Geologic Investigations). For the whole fault zone, average surface displacement may approach 3 m. In the Bullion Mountains the surface faulting is relatively simple, with most displacement occurring on a single trace, or closely spaced parallel traces. However, to the north and south of the Bullion Mountains faulting patterns become quite complex and slip is distributed across broad zones of faults with various orientations.
On October 24, helicopter and ground reconnaissance teams found additional rupture within a zone that occurs southwest, and parallel to, the Bullion fault (shown in pink on the figure). The continuity and amount of right-lateral slip associated with this zone is presently unknown. In addition, small cracks, with no consistent lateral displacement have been observed near Lavic Road NE of Lavic Lake, and near Hector Mine (~ 8 km NW of Lavic Lake).