No crustal fault is an island, seismologists are learning. Last weekend's Hector Mine earthquake, which rocked the desert 160 kilometers northeast of Los Angeles, seems to support the idea that faults feel what happens to their neighbors. The tremblor appears to have been triggered by the magnitude 7.3 Landers quake of 1992, which struck 160 kilometers to the east of Los Angeles. "There's clearly a relation" between the Landers and Hector Mine quakes, says seismologist Lucile Jones of the U.S. Geological Survey (USGS) in Pasadena, California, "but we clearly do not understand that relation."
Geologists have speculated that faults can reach out and touch one another because earthquakes redistribute stress. When a fault ruptures, it reduces stress in broad zones to either side; their extent can be calculated from the way the fault broke. Great earthquakes like the 1906 San Francisco quake reduced the stress over great swaths along hundreds of kilometers of the San Andreas fault, damping seismic activity in those areas for decades (Science, 16 February 1996, p. 910).
But stress can actually increase in areas beyond either end of a ruptured fault. The Landers rupture was much shorter than the 400 kilometers of a great quake and produced prominent lobes of heightened stress across the Mojave Desert to the north and south across the San Andreas near Palm Springs and Riverside. One southward lobe, as calculated by seismologist Ross Stein of the USGS in Menlo Park, California, and his colleagues, seemed to trigger the magnitude 6.2 Big Bear quake 3 hours after Landers struck 35 kilometers away.
Now, the other shoe seems to have fallen. Early last Saturday morning, 40 kilometers of a previously unnamed fault, now dubbed the Lavic Lake fault, broke across the Twentynine Palms Marine Base. The quake's epicenter and much of the rupture lie in what Stein and his colleagues calculate was one of the two northern lobes where the Landers quake intensified the stress. Apparently "if you jack up the stress on a fault, you get a higher rate of earthquakes, big ones and smaller ones," says Stein.