In an explosive reaction, a large earthquake can set off a nearby volcano that is on the verge of erupting, geophysicists have found. An analysis of quakes and eruptions during the past several centuries revealed about 20 pairs that struck within 750 kilometers of each other, according to a report in tomorrow's issue of Nature. The researchers suspect that seismic waves shake loose a cascade of gas bubbles within chambers of molten rock, causing magma to burst like a victor's magnum of champagne.
Scientists have linked only a few earthquakes and eruptions. For instance, Charles Darwin noted that two volcanoes erupted in Chile on the same day as a big quake there in 1835. Japanese seismologists proposed in the 1970s that seismic waves could trigger distant eruptions, but few geophysicists elsewhere thought that the fleeting waves could rattle a far-off volcano. Then the magnitude 7.3 Landers earthquake in Southern California in June 1992 changed that thinking. The temblor spawned no eruptions, but it set off small quakes all across the western United States, especially near active volcanic and geothermal regions.
To see whether quakes of this magnitude might have triggered past eruptions, geophysicists Alan Linde and I. Selwyn Sacks of the Carnegie Institution of Washington in Washington, D.C., scrutinized historical records of earthquakes and volcanoes dating to the 16th century. They found eight instances in which quakes of magnitude 8 or higher were followed the same day by at least one eruption within 750 kilometers. Nine other eruptions clipped the heels of magnitude 7 to 7.9 quakes, whose epicenters were within 200 kilometers of the volcanoes. "The chances of this happening at random are about one in a million," Sacks says. "The correlation really sticks out."
Sacks and Linde theorize that the seismic waves, which subside quickly with distance, still pack enough punch to dislodge marble-sized gas bubbles from the walls and floors of a magma chamber. As the bubbles rise, they pressurize the top of the chamber until it explodes. "That mechanism could work for a volcano already near a critical state," says geophysicist David Hill of the U.S. Geological Survey in Menlo Park, California. "It advances the clock on a process that has been ticking in a slow way."