Earthquakes Can Trigger More Earthquakes, Experts Say

Brian Handwerk
National Geographic Channel
for National Geographic News
May 5, 2005

On TV: Catch Explorer: Violent Earth on the National Geographic Channel (U.S.) Sunday, May 8, at 8 p.m. ET/PT.

Can one earthquake cause another? A developing theory holds that quakes can pressure highly stressed fault lines and trigger subsequent seismic events. If correct, so-called stress triggering theory could help scientists pinpoint areas where earthquakes are imminent.

The basic idea is that earthquakes conduct "conversations." When a quake ruptures one fault, seismic stress shifts to neighboring faults, adding pressure that can trigger yet another quake.

"Generally a rupture will [reduce] the stress in the fault that's [ruptured], but will increase it in other places," said Ross Stein, a geophysicist with the U.S. Geological Survey's Earthquake Hazards Team in Menlo Park, California. "All other things being equal, we'll get more seismicity [quake activity] in those places."

Scientists may never be able to accurately forecast earthquakes. But Stein and other proponents of stress-triggering theory say it could become a useful tool for identifying the specific fault areas that are most vulnerable to future seismic events.

The theory rests on the notion that faults can be extremely sensitive to stress. In fact, Stein believes that some faults can be ruptured by less pressure than that needed to inflate a car tire.

Rethinking Aftershocks

The general idea behind the theory blossomed after a magnitude 7.3 earthquake hit Landers, California, on June 28, 1992. Three hours later a magnitude 6.5 quake occurred in Big Bear, California—some 25 miles (40 kilometers) from Landers.

While aftershocks are common during earthquake events, they were generally thought to occur along the same fault lines as the initial earthquakes. The Big Bear quake, however, was centered on a different fault altogether.

Stein and others went to work on the tantalizing possibilities raised by the events. They found that the Landers quake had increased stress in the Big Bear area, where the secondary quake occurred.

Many aftershocks from the Landers quake also occurred in areas where researchers determined stress had risen. But aftershocks were far less common in areas where they calculated stress had dropped.

Those connections marked a significant departure from more traditional views of earthquakes and aftershocks, namely that they occur along the same fault.

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