(2 of 4)
About 1,300 km (800 miles) long, the San Andreas Fault system separates two sections of the earth's crust known as plates. Like giant rafts, these plates glide across an expanse of superheated rock, viscous as tar, that surrounds the planet's molten outer core. At the rate of nearly 5 cm (2 in.) a year, the Pacific plate to the west of the San Andreas is slowly pushing north, past the North American plate on the east. One possible result: 60 million or so years from now, a sliver of the California coast that includes the megalopolis of Los Angeles could become beachfront property in Alaska.
Getting there, however, will not be fun. The slip of the plates is not constant along the fault. The southern San Andreas bends like a river and splits into multiple branches. Because of this contortion, the Pacific and North American plates cannot slip in a straightforward way but must strain against each other like two sumo wrestlers. The battle of the plates has created numerous smaller fault lines along the San Andreas, giving the region the look of a smashed windshield. Over the millenniums, the Mojave shear zone to the east may offer a path of less resistance to the giant plates and replace the San Andreas as a new plate boundary, suggests geophysicist Amos Nur of Stanford.
Only four years ago, scientists gave the stuck plates along the southernmost section of the San Andreas a 40% chance of snapping sometime in the next 30 years. At the same time, they warned that a rupture of this part of the fault & could trigger earthquakes along neighboring segments, possibly as far west as San Bernardino and nearly as far north as Bakersfield. Result: the long-feared Big One -- an earthquake of magnitude 8, five times as powerful as Landers -- on the doorstep of the populous Los Angeles Basin. Now, in the seismic spoor of the Landers earthquake, scientists have found reason to suspect that the timetable for this disaster may have been fast-forwarded.
A fateful chain reaction, seismologists believe, started in April, when an earthquake of 6.3 magnitude rattled the vicinity of Palm Springs and Joshua Tree National Monument. On a map, the fault that was then broken looks like a shotgun taking dead aim at Landers, and in fact it was. Two months later, a minor earthquake started on a fault with no name. For a few seconds, this temblor rattled at a magnitude of 3. Suddenly, seismometer readings soared as the fracture unzipped a sequence of larger faults nearby. Then three hours after the Landers earthquake shivered to a stop, a 6.6 aftershock terrified the environs of Big Bear Lake, collapsing chimneys and toppling buildings.
Why Big Bear? In recent weeks research teams at the U.S. Geological Survey in Menlo Park have put this question to two different computer models. The results, while differing in detail, are strikingly similar. Before the effects of the Landers earthquake are taken into account, neither model flags the region around the Big Bear fault as particularly menacing. But as soon as scientists factor in the degree of ground movement and its direction, it pops up on their computer screens, color-coded red for danger.