With memories of last January's devastating earthquake fresh in their minds, Los Angeles residents may not feel geologically lucky. But they should, suggest results of three new studies.
Judging by the known faults lacing the crust beneath the city, geologist James F. Dolan and his colleagues conclude that Los Angeles has been dodging seismic bullets ever since its birth. Although stress has accumulated steadily under the metropolitan region, the city has suffered no large earthquakes and only two moderate ones in the last 2 centuries, they report in the JAn. 13 Science. In other words, Los Angeles has accrued a quake debt that must eventually come due.
"The basic conclusion is that Los Angeles is likely to face either more numerous or larger earthquakes in the future," says Dolan, a researcher with the Southern California Earthquake Center in Los Angeles.
Two other new studies reach a similar conclusion about the city's quake risk.
Many past hazard assessments for southern California have focused on the threat of great, magnitude 8.0 quakes from the San Andreas Fault, which runs within some 50 kilometers of Los Angeles. But the magnitude 6.7 Northridge quake on Jan. 17, 1994, drew attention to faults directly under the city. Causing more than $15 billion in damage, Northridge demonstrated that moderate quakes beneath a heavily populated region can cause at least as much damage as a great San Andreas quake. The moment magnitude 6.8 killer tremor near the Japanese city of Kobe this week offered a similar lesson.
The energy for earthquakes in Los Angeles, and for quakes in much of California, comes from an ongoing, grinding collision between the Pacific and North American plates. The movement slowly compresses the Los Angeles basin in a north-south direction, straining the crust until it finally breaks along faults.
Dolan and his colleagues analyzed the quake risk Los Angeles faces by assessing the six major fault systems known in the area. From the fault dimensions, they calculated the size of potential earthquakes. To estimate the frequency of such quakes, they looked at the geologic slip rates -- how quickly rock on one side of a fault has slid past rock on the other side over the millennia.
According to their calculations, Los Angeles has stored enough strain energy in the last 2 centuries for 17 Northridge-size earthquakes. Yet only two earthquakes of this size have occurred, in 1971 and 1994. That leaves enough energy for 15 magnitude 6.7 jolts sitting beneath the city. …