Water can increase the pressure in a hair-trigger fault line, and push it over the edge.Punchstock

A spate of rain is all it takes to set off some earthquakes. That's what a team of German geologists has discovered after monitoring swarms of tiny tremors in the mountains of Bavaria.

Sebastian Hainzl of the University of Potsdam and his colleagues say that the rise in water pressure within porous rocks as rain soaks into the ground can start quakes on hair-trigger faults. "Tiny changes can have big effects," says Hainzl's coworker Toni Kraft of Ludwig-Maximilians University in Munich.

The idea that changes in pore-water pressure can induce earthquakes is well established. And seismic activity can be seasonal, perhaps because of rainfall variations.

But the link has never been firmly established, and it was generally thought that much larger water flows are needed than those produced by rainfall.

Reservoir filling can trigger quakes, for example, either because of the weight of water or its permeation into the rock. One of the most notorious examples happened in 1967 in western India, where the reservoir created by the Koyna Dam, completed in 1962, is thought to have caused a magnitude-7 quake that killed 200 people.

Pressure changes caused by melting snow may also trigger quakes (see 'Melt may trigger quakes'). But rainfall causes much smaller changes in the mass and flow of ground water.

Wet days in Bavaria

In 2002, Hainzl and his colleagues monitored quakes beneath Mount Hochstaufen, a 1,775-m peak in the Staufen massif of Bavaria, southeast Germany, which typically experiences over a thousand tiny earthquakes each year. They found there was more seismic activity in the summer, when the weather is wetter.

To test whether the two things were related, the researchers calculated how the water pressure in the rocks would have varied based on the daily observed rainfall, and then used the resulting changes to estimate the earthquake rate. The predictions showed a good match with the number and size of tremors from day to day.

In particular, there were heavy rains in March and August. "After that, the seismicity increased from just one or two to up to 40 tremors a day," says Kraft.

On the brink

The researchers think that the faults in this region must already be poised on the brink of sliding, so that a small push sets them off.

Could this happen in regions where the quakes can be bigger and more dangerous? "I would think so," Kraft says. He thinks the results support the previous tentative links between seismic activity and rainfall elsewhere in the world, suggesting that the crust might be similarly close to rupture in those places.

Mark Zoback, a geophysicist at Stanford University in California, agrees. "In most places, the shallow, brittle crust is close to failure," he says. "The effect of rain could be just enough to send it over the edge." But he points out that for deeper earthquakes, the water could take many years to penetrate, making it hard to be sure about cause and effect.

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• References

  1. Hainzl S., et al. Geophys. Res. Lett., 33. L19303 (2006).