'This Was a Warning'

The 5.4 magnitude earthquake that struck near Los Angeles yesterday just before noon was felt as far east as Las Vegas and as far south as San Diego. Since the quake, whose epicenter was about 28 miles southeast of Los Angeles near the affluent suburb of Chino Hills and 8.5 miles beneath the Earth's surface, there have been nearly 50 aftershocks, most of them small, according to the U.S. Geological Survey.

Although the quake was one of the strongest to hit urban southern California since the deadly 6.7 magnitude Northridge quake in 1994, it didn't cause any severe damage or major injury. So southern California residents are exhaling with relief—that for now, they've eluded The Big One.

But seismologists, who've concluded that this jolt was not tied directly to any of southern California's major fault lines, insist it could have been much more destructive and that it was just a reminder that the big one is indeed still looming. Graham Kent, a seismologist and geophysicist at the Scripps Institution of Oceanography in La Jolla, Calif., studies the complex series of fault lines that run beneath California's surface. NEWSWEEK'S Jamie Reno spoke with Kent about this quake and the threat that possible future quakes pose to southern California. Excerpts:

NEWSWEEK: First of all, can you tell us which fault produced this particular earthquake?
Graham Kent: Geographically, it's located in a messy area, near the Elsinore Fault near downtown Los Angeles, but not truly on that fault. Elsinore is relatively major fault line, and runs into the larger faults such as San Andreas. The smaller faults die out and meld into a whole melange of what we call thrust faults. This is an area under a lot of compression, and so in some ways the thrust faults have more of a pop, that's why we felt this quake so widely. The L.A. basin is riddled with thrust faults.

Have there been other earthquakes in this particular area? Did this quake take seismologists by surprise?
The Whitter Narrows quake in 1987 was relatively close, and it was a little bit bigger. But it was the same type of quake, a thrust fault. If you looked at the image of faults of the L.A. basin, you'd be amazed, there are a lot. We tend to think about San Andres and San Jacinto, the larger faults, but there are any number of thrust faults from large to small that can rupture in the Los Angeles area. This one is in the middle as far as size. It's significant but a thrust fault can be up into the sevens. The San Fernando Valley earthquake was in the high sixes, the Northridge was in the high sixes. That was a thrust event as well, the same type of thing.

We've seen about 50 aftershocks already, the largest of which so far has been a 3.8 magnitude quake. Is that typical?
Yes, that's not a surprise for any earthquake. If there weren't dozens and dozens of aftershocks by now, that would be the bigger story.