A powerful earthquake splits the California desert floor, killing a toddler and crumbling homes. Years later and a dozen miles away, another huge tremor on a different fault rocks the area.

Scientists now believe the two events were related — and they are beginning to understand how.

In a study published in Thursday's issue of the journal Nature, researchers say they have directly measured for the first time how strong seismic shaking can weaken an adjacent and unrelated geologic fault.

"It's a very interesting discovery," said Christopher Scholz of the Lamont-Doherty Earth Observatory of Columbia University, who did not participate in the study. "We already know that earthquakes trigger other earthquakes on other faults. This provides some additional information that may tell us how that happens."

The study compares two earthquakes in the Mojave Desert. The 1992 Landers quake registered a magnitude 7.3, killed one person and caused $100 million in damage.

After the quake, geologists buried seismometers into the fault and set off explosions in boreholes drilled more than 100 feet down. The explosions simulated earthquake tremors.

Five years of observations of the seismic waves showed that the shattered crust was slowly repairing itself.

Then in 1999, the Hector Mine quake struck. The magnitude-7.1 quake occurred on a different fault about a dozen miles from the Landers quake. Hector Mine caused little surface damage because it was in a more remote area.

But seismic measurements showed the Hector Mine quake weakened the Landers fault and interrupted its self-repair.

"We were watching the Landers fault heal," said John Vidale, a geophysicist at the University of California, Los Angeles, and the study's lead author. "After the Hector Mine earthquake, we could actually see that the Landers fault had become weaker."

Because geologic stress can move back and forth, the researchers speculate the Landers quake probably helped to weaken the Hector Mine fault before it finally broke in 1999.

"It appears that one earthquake can slightly weaken the faults around it, which may make the surrounding faults more likely to have their own earthquakes," Vidale said.