Earthquakes were once thought to keep to themselves, striking a particular fault without regard to what other faults were doing. Now a group of geophysicists is suggesting that big quakes can trigger other quakes thousands of kilometers away and decades later. According to calculations presented in today's Science, great earthquakes that struck the far North Pacific in the 1950s and '60s could have triggered a pulse of seismic activity that swept California in the 1980s.
Researchers have long recognized a potential transmission route for communication among faults: the thin layer of soft but solid rock at depths of 80 kilometers or more, called the asthenosphere. Tectonic plates glide on the soft asthenosphere but at their edges the plates stick to each other, build up stress, and eventually jerk forward in earthquakes. Four great earthquakes struck along the Aleutian Islands and the Kamchatka Peninsula between 1952 and 1965, where the Pacific plate is diving beneath the North American plate. Each quake triggered flow of the underlying asthenosphere that rippled outward. This flow wave could transmit the stress induced by the quakes.
In a model created by geophysicists Fred F. Pollitz and Roland Bürgmann of the University of California, Davis, and seismologist Barbara Romanowicz of UC Berkeley, the stress wave generated by the great quakes moved southward across the Pacific in time to trigger surges of quakes from off British Columbia to southern California in the 1980s. To the north, the wave could have triggered quakes in the Arctic Ocean in the 1970s. That tight fit--as well as the types of quakes in the surges--suggest that the stress wave was responsible, say Pollitz and his colleagues.
"The whole thing seems to hang together," says seismologist Frank Press of the Washington Advisory Group in Washington, D.C. But others point out that the correlation of the passing wave with a flurry of seismicity could be due to chance. Researchers who measure subtle deformation of the crust should be able to test the team's bold idea by recording the wave's departure from California during the coming decade.