By Sid Perkins
Using the locations of moderate-sized quakes to estimate where “The Big One” will eventually strike may not work for all regions, a new study reveals.
Many researchers assume that small-scale seismic activity reveals where stress is building up in the Earth’s crust — stress that can cause larger quakes in the future, says Mian Liu, a geophysicist at the University of Missouri in Columbia. However, Liu and Seth Stein of Northwestern University in Evanston, Ill., report in the Nov. 5 Nature, many moderate-sized temblors that occur far from the edges of tectonic plates could be merely the aftershocks of larger quakes that occurred along the same faults decades or even centuries ago.
Most large earthquakes occur along the edges of tectonic plates, where stress and strain accumulate as large masses of fractured crust jostle and scrape past each other. But major temblors can also strike fault zones in continental interiors thousands of kilometers from such interfaces. Such quakes are less frequent and therefore much less predictable. “Intraplate quakes don’t follow a single pattern,” Liu says.
Stein and Liu analyzed earthquake data gathered worldwide. For major quakes that occurred where the sides of a fault moved past each other at average rates of more than 10 millimeters per year — as the two sides of many tectonic boundaries do — aftershocks died off after a decade or so. But for faults where the sides scraped past each other at just a few millimeters per year, aftershocks lasted about 100 years, the researchers reported. The longest series of aftershocks, some which have lasted several centuries, were triggered by quakes that occurred in continental interiors along slow-moving faults.