Vidale's research marks the first time scientists have shown that one fault has been damaged by the shaking and stress from an earthquake on a different fault, he said. The rock within approximately 100 yards of a major fault slip surface is highly fractured, and is therefore especially vulnerable to such damage, Vidale said.

The research implies that a large earthquake may slightly loosen up a system of faults around it, raising the danger of further earthquakes, said Vidale, who is also director of UCLA's Institute of Geophysics and Planetary Physics.

"We were watching the Landers fault heal after it broke in 1992," Vidale said.

"In measurements we took in 2000, after the Hector Mine earthquake, we saw that instead of continuing to heal, the rock had become less strong. We could actually see that the Landers fault had become weaker.

"Prior to that, we had four measurements from the five years before the Hector Mine earthquake that revealed progressive recovery -- each time stronger than the previous measurement. It was serendipitous that we captured a magnitude seven earthquake 10 kilometers away."

Vidale and Li's measurements were the first to detect that earthquake rupture measurably weakens a fault, which then heals over time.

Vidale and Li measured how fast seismic waves travel through the rock, which gives an indication of the rock's strength. (The speed by which the seismic waves travel is proportional to the stiffness of the rock; as fractures that weaken the rock heal, the velocity increases.)

The Landers fault is also in the Mojave Desert, less than 10 miles away from the Hector Mine fault.

"It could be that the kind of damage the Hector Mine earthquake caused on another fault is a reason why one earthquake leads to another earthquake," Vidale said. While there have been indications of activity, such as slippage, on faults following distant earthquakes, this is the first time scientists have seen evidence that an earthquake damages a different fault line.

"We didn't expect that the neighboring earthquake would damage the Landers fault at all, so we were surprised when the measurements showed that it did, and when the measurements held up over multiple measurements."

The Landers fault has again resumed healing, Vidale said.

Vidale studies approximately the upper mile of the fault. He and Li hope to extend their results to a 3?10 mile depth, where the most energy is released in earthquakes.

Geologists have observed that earthquakes are often followed by earthquakes on a neighboring fault segment, but did not understand how this clustering occurs, Vidale said.

Major California faults, including the San Andreas, have several parallel faults. Although it is speculative, Vidale said the Landers earthquake may have weakened the Hector Mine fault and allowed the 1999 earthquake to strike.

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