Finding faults: geology

Tim Henstock spent a few nights not long ago setting off minor explosions and firing air guns across a swath of northern California. Henstock, a geologist at Rice University in Houston, was making a seismic portrait of Earth's crust beneath the San Andreas Fault. He found that the fault has much deeper roots than geologists had suspected, plunging 15 miles through the crust all the way into Earth's mantle.

The San Andreas Fault system marks where the northbound Pacific plate grinds past the southbound North American plate. In northern California the system consists of three roughly parallel hurts: me San Andreas, the Maacama, and the Bartlett Springs. Henstock was trying to find out if the three faults were connected, as some earthquake models have assumed.

To map the faults, Henstock set off a series of seismic waves, both offshore and on land. From a ship, he fired big air guns underwater. The pulses from the guns generated artificial seismic waves on the seafloor. As seismic waves travel through the crust and mantle, different types of rock alter the waves' speeds. The seismic waves also reflect and refract in telltale patterns, depending on the type of rock they travel through. Henstock recorded these seismic waves with sensitive receivers placed along a 60-mile stretch of seafloor. On land, he set off explosive charges and recorded the resulting seismic waves. From the speed and direction of the waves he recorded, Henstock was able to map the rocks under the fault system.

At the ragged boundary between the crust and the mantle, Henstock found a few places where the crust penetrates as much as two miles into the upper mantle. These crustal dips into the mantle line up almost exactly with hurts in the upper crust near the surface, 12 to 15 miles above. "We're suggesting that the two are correlated," says Henstock.

Although faults in Earth's crust are apparently deeper than anyone had suspected, Henstock says that earthquakes originate only in the upper crust. As crustal plates slide past one another, the warm, plastic rocks near the mantle deform smoothly, he says. But the colder, stiffer rocks higher in the crust build up strain and suddenly release it, causing an earthquake.

While his map won't soon help predict earthquakes, the detailed information is a boon to researchers. "A fair amount of our understanding of the way hurts behave has come from physical models," says Henstock. "One very important parameter that's been missing from these models is knowing in detail the structure you're starting off with. So ultimately this will probably help us understand why particular patterns of earthquakes happen."

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