Do Stress Waves Kick Off Quakes?

SAN FRANCISCO--Massive earthquakes along deep-sea trenches may spawn slow-moving "stress pulses" under the ocean floor that trigger other quakes decades later and thousands of kilometers away, scientists said here yesterday at the fall meeting of the American Geophysical Union. The researchers found a tentative correlation between several huge tremblers in Alaska more than 30 years ago and a marked rise in California's recent seismic activity. However, the purported effect is so subtle that other researchers are skeptical.

For years, seismologists believed that most earthquakes happen in isolation. But the 1992 magnitude-7.4 Landers earthquake in southern California took place in concert with a series of smaller quakes in the region, both before and after the main shock, revealing a complex exchange of stress among neighboring faults. Some seismologists even hypothesized that the largest earthquakes of all, in oceanic subduction zones, might trigger other quakes part way around the globe.

The new study offers tantalizing hints supporting the idea. Four major earthquakes shook the North Pacific between 1952 and 1965, as oceanic crust plunged into a deep-sea trench off the Aleutian Islands, tugging on the ocean floor to the south. Geophysicists Fred Pollitz and Roland Burgmann, of the University of California, Davis, think the tension could have stretched the rest of the Pacific floor like a rubber band. They say the crust and upper mantle then began to contract again, relaxing the tension, beginning at the subduction zone. This wave of acceleration rippled southward at more than a hundred kilometers per year.

Reaching the California coast in the mid-1980s, so the theory goes, the pulse still packed enough punch to disturb the delicate balance of several faults perched on the verge of rupture. Burgmann says that he and Pollitz found a general trend of large quakes from north to south in California over the last decade, in keeping with the strain-wave scenario. However, the researchers don't attribute any particular earthquake to the pulse; "only when we look at all of the events do we see a broad correlation."

"This notion is intriguing and worth pursuing, but there are many alternatives," says geophysicist Steven Cohen of the Goddard Space Flight Center in Greenbelt, Maryland. The big question, he adds, is how earthquakes get started in the first place. Cohen also isn't convinced that layers in the crust and uppermost mantle can transmit a pulse as efficiently as Pollitz and Burgmann claim.