CAN one shake trigger another? Big earthquakes cause others in distant places at least 9 per cent of the time, according to a new statistical analysis.
Tom Parsons of the US Geological Survey in Menlo Park, California, surveyed seismic activity going back to 1979 on every continent except Antarctica. Over the period, there were 260 earthquakes of magnitude 7 or greater. Twenty-four were followed by small quakes on separate fault systems. "It's a small hazard, but there is a risk," he says.
Parsons was keen to see if any tremors of magnitude 5 or larger – which can cause serious damage – could be triggered by quakes far away. He found seven events that might qualify, but all happened at least 9 hours after seismic waves from the possible trigger quake had passed through. "We still can't say whether they're linked," he says.
The mechanism by which one quake might trigger another after such a long delay is something geologists can only speculate about. The planet's plumbing could play a role: there is evidence that seismic waves can affect the water table at remote locations, and changes in water pressure can trigger quakes.
Parsons's results were presented last week at the Seismological Society of America annual meeting in Salt Lake City, Utah.
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