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Earthquakes Can Trigger More Earthquakes, Experts Say

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

Instead, stress-triggering theory holds that such aftershocks are simply neighboring earthquakes in areas where stresses have increased.

Kate Hutton, a seismologist at Caltech in Pasadena, California, said the theory has changed her view of "the catalog of earthquake history."

For example, the great San Francisco earthquake of 1906 was followed the next day by a large earthquake in California's Imperial Valley.

Hutton says that, going forward, new seismic activity should provide ample opportunity to study stress-triggering theory.

She points to the Sumatra area of Indonesia as "a likely candidate for research." The area witnessed a magnitude 9.3 quake on December 26, 2004, which triggered the Indian Ocean tsunami disaster. That quake was followed by an 8.7 temblor on March 28, 2005.

Seismic Wave

Thomas Henyey chairs the Department of Earth Sciences at the University of Southern California in Los Angeles. He says that the orientation of a fault—say, north-south or east-west—can affect how a fault responds to seismic stresses.

"Depending on the orientation of a fault, it may be that the increase in stress produced by a past earthquake will move that fault closer to failure," he said. However, in a fault at another orientation, "the change in stress could actually lengthen the time to the next earthquake or have no effect at all."

The orientation issue is not as random as it may seem.

"Here in Southern California, and I suspect in many places, you don't typically find a situation where faults have all different kinds of orientations," Henyey said.

"Over the long term, [a regional stress field] produces faults of similar orientation," he added. "So when [a fault is acted upon], it almost always increases the stress on the neighboring fault, because if they have the same orientation, it increases the stress."

The more tangible effects of an earthquake—namely the earthshaking seismic wave that rattles cupboards or collapses buildings—may itself transfer enough stress to trigger subsequent quakes.

"After 2002's 7.9 magnitude quake at Denali Fault [in Alaska], the Wasatch Fault in Utah lit up, Yellowstone lit up," Stein, the U.S. Geological Survey geophysicist, said. "Both places were bathed in small quakes."

"The permanent stresses associated with a quake 2,000 miles [3,200 kilometers] away are zip. They're nothing," he added. "But temporary stress is different. As the waves travel, they seem to trigger little quakes, and some can last for a while."

All things being equal, it seems a simple proposition that the areas of greatest seismic stress would have the greatest seismic activity. But because the Earth's crust isn't homogenous, all things usually aren't equal.

Scientists are developing more complicated models to account for mitigating factors, such as the delays between quakes that allow some degree of stress decay.

Proponents of stress triggering say the theory is gaining ground.

Stein notes, for example, that a decade ago many seismologists discounted the stress-triggering theory, because the stresses involved were so small. But today, Stein said, "Most people would say [stress triggering] is part of the story, a part of the process by which earthquakes are triggered and controlled."

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