The Merapi (pictured) and Semeru volcanoes blew their tops in quick succession.

(map) Harris and Ripepe, Geophysical Research Letters 34 (2007)

Shaken, Volcanoes Stir to Life

Like a dark storm cloud on the horizon, an earthquake can be a harbinger of bad news. A new study provides the strongest evidence yet that quakes can trigger volcanic activity. Knowledge about how and why such events occur could help predict impending eruptions, warn populations, and potentially save thousands of lives.

Both earthquakes and volcanoes arise from deep within Earth's surface. Quakes are caused by collisions between tectonic plates floating on Earth's mantle, which is home to the lava that volcanoes spew. It's perhaps not surprising then that the two geological events could be linked. Indeed, several events over the past century have provided fodder for the theory. In 1975, for example, Hawaii's Kilauea Volcano erupted less than an hour after an earthquake struck a few kilometers away. Similarly, Chile's Cordon Caulle volcano blew its top 2 days after a major earthquake that was centered 240 kilometers away. Still, there was no direct scientific evidence that quakes spark volcanoes.

That changed in May of last year--and quite by accident. Geologists Andrew Harris of the University of Hawaii at Manoa and Maurizio Ripepe of the University of Firenze in Italy were studying two active Indonesian volcanoes, Merapi and Semeru, while gathering data on global hot spots. The team was employing so-called thermal mapping, which uses satellite imagery to monitor the temperature and pace of lava oozing from the volcanoes. After 16 days of observations, a 6.4-magnitude quake struck the island of Java, 50 kilometers from Merapi, 280 from Semeru. Just 3 days later, the lava from both volcanoes was twice as hot and flowing twice as fast. This enhanced activity lasted for more than a week, the team reports in the January issue of Geophysical Research Letters.

The volcanoes are too far apart to influence each other, Harris says, which makes him believe that the Java quake was the trigger for the vigorous outpouring of lava. One possible mechanism is what Harris calls "the toothpaste model." Here, underground stresses reverberating from earthquakes and their aftershocks cause magma to be squeezed upward, shooting out of a volcano's crater.

"It's the first quantitative measure of an increase in activity from thermal mapping," says Emily Brodsky, a seismologist of University of California, Santa Cruz. "It's a real step forward."

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