Little Quakes Precede A Big One?

Typically, small earthquake swarms have been considered as relieving the stress on a fault. In Italy, only 2 percent of small swarm events had preceded a bigger earthquake, which is why the magnitude-6.3 quake in L’Aquila, Italy, in 2009 came as a surprise even to the seismologists, who now face jail time for downplaying the risk.

But what if there was a way to tell when swarms indicated something more dangerous approaching? Stanford geophysics Professor Paul Segall reporting at the meeting of the American Geophysical Union in San Francisco is using computational models to search for the signature events that may precede a major earthquake. He is relying on data from the Pacific Northwest where the last earthquake to shake the region with an estimated 8.7–9.2 magnitude occurred in 1700 and produced a tsunami that reached Japan.

“You have these small events every 15 months or so, and a magnitude 9 earthquake every 500 years. We need to known whether you want to raise an alert every time one of these small events happens,” he says. “What our calculations have shown is that ultimately these slow events do evolve into the ultimate fast event, and it does this on a pretty short time scale.”

Still, Segall says they don’t have a matching signature just yet. They are continuing to look for significant differences in the numerical simulations between the average slow slip event and those that directly precede a big earthquake.