Small earthquakes may not predict larger ones

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Quakes far from tectonic plate boundaries may simply be aftershocks of ancient temblors
By Sid Perkins
Web edition : Wednesday, November 4th, 2009

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. “Intercontinental 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.
Continental quakes have this effect because their energy is stored longer. When major quakes occur, some of the energy that’s released gets stored in the viscous material of the Earth’s lower crust and upper mantle, Liu says. Later, that stored energy is often released in aftershocks. Along fast-moving tectonic interfaces, where stresses build quickly and seismic activity is much more frequent, the stored energy is readily released. But in continental interiors, typically far from plate edges, stress builds slowly and quakes are infrequent, so aftershocks can echo for centuries.
The Midwest’s New Madrid Fault Zone is one such region, Liu says. Small- to moderate-sized temblors still occasionally rock this area, where four major quakes occurred between December 1811 and February 1812. But the recent quakes bear the hallmarks of aftershocks: For instance, they’re happening along the same areas of the fault ruptured by the original shocks, and they’re occurring less frequently as time progresses.
Identifying modern-day quakes as aftershocks “doesn’t make life much easier,” Liu says. Scientists may be able to better predict earthquakes using tools such as GPS equipment to discern movements indicating that stresses are building up in the Earth’s crust. Recent field studies suggest that stress isn’t accumulating along the New Madrid Fault Zone, he notes.
“Aftershocks don’t help you predict where the next big shock can occur,” says Tom Parsons, a research geophysicist with the U.S. Geological Survey in Menlo Park, Calif. In midplate regions where repetitive cycles of earthquakes can take millennia to unfold, forecasting when and where the next big quake will occur is akin to predicting a full year’s weather based on watching conditions during one week in January, he notes in a commentary in the same issue of Nature.