Traffic-light system can predict repeat earthquakes
Detecting the size of aftershocks could help manage them. Richard A Lovett reports.
Earthquake researchers believe they have found a “traffic-light” style warning system that can determine if a big earthquake is a prelude to an even larger event, or is itself the main shock.
“Usually a big earthquake is followed by many other events whose magnitude and number decreases with time,” says Laura Gulia of the Swiss Seismological Service in Zurich. “But sometimes a bigger event happens.”
Historically, she says, it has been impossible to determine which earthquake is the main shock until after the entire sequence is over – a dangerous and frustrating situation for people wanting to know when it’s safe to resume their normal lives.
“You can’t sit down and look at a seismometer and say this is a main shock and this is a foreshock and this is an aftershock,” adds Emily Brodsky, an earthquake physicist at the University of California, Santa Cruz. “An earthquake is an earthquake.”
To resolve this conundrum, Gulia and Stefan Wiemer, also of the Swiss Seismological Service, turned to the entire swarm of earthquakes that always accompanies any large one - a family that can include thousands of temblors of varying sizes.
Their work appears in the journal Nature.
Normally, the pattern of smaller and larger earthquakes follows a well-known rule: “In general, we get ten times as many magnitude threes as fours, ten times as many magnitude fours as fives, and so on up,” says Brodsky, author of a related commentary.
To see if these ratios changed during major earthquake swarms, Guila and Wiemer looked at the pattern of earthquakes before and after two large earthquakes, one in Italy (magnitude 6.6) and one in Japan (magnitude 7.3).
Each was preceded by a large but slightly smaller earthquake, and in each case, the question was: how do you know that the first earthquake was a foreshock, while the second was the main event, not to be followed by something even larger?
They discovered that in each case the swarm of smaller earthquakes preceding the shock that proved to be the main event had a higher-than-normal proportion of larger earthquakes – a sign of mounting seismic stress. The main event, however, was quickly followed by the opposite pattern, with an unusually small fraction of larger earthquakes – a sign that things were calming down.
To test this pattern, the researchers turned to seismic records from 58 prior earthquakes, magnitude 6.0 or larger, discovering that their protocol correctly predicted the main shock 95% of the time.
Based on that, they developed a near-real-time warning method that functions on what they call a traffic-light system.
“Red” means that the largest earthquake is yet to come. “Orange” means maybe yes, maybe no. “Green” means the main shock has occurred and the magnitude and frequency of aftershocks can be expected to decrease.
All of this is based on the ratios of small to large earthquakes in the ongoing swarm of smaller earthquakes.
“If you haven’t seen that increase yet [in the proportion of small earthquakes],” says Brodsky, “you’re still waiting for the big one.”