Potential signs of an underground killer.

Potential signs of an underground killer

Many people think of California as synonymous with earthquakes. But the largest quakes in the history of the contiguous United States have actually hit the supposed stable heartland, far from the edges of the crustal plate that carries North America.

Geologists fear that such large "intraplate" earthquakes will strike again, yet they have a hard time predicting where the shocks will occur. A new study points to a possible way of identifying earthquake-prone areas even in the middle of a plate.

Robert M. Hamilton of Reston, Va., and Walter D. Mooney of Menlo Park, Calif., both with the U.S. Geological Survey, examined data collected in 1980 during a series of experimental explosions set off in the New Madrid fault zone, where Missouri, Kentucky, Tennessee and Arkansas meet. In late 1811 and early 1812, a series of three shocks greater than magnitude 8 struck this area, shaking it so violently that scaffolding rattled at the U.S. Capitol building in Washington, D.C., more than 700 miles away.

Hamilton and Mooney analyzed recordings made by seismometers surrounding the experimental blasts. They found that seismic waves from the explosions weakened considerably when passing through active parts of the fault zone -- areas that regularly produce small earthquakes. The waves traveled much farther when they went through inactive parts of the New Madrid fault zone, the researchers report in the April 20 SCIENCE. Previous studies had suggested a general relationship between quakeprone regions and wave attenuation, or weakening, but they did not use experimental blasts to look at specific faults in detail.

Noting that repeated earthquakes can fracture rocks in a fault, the researchers propose rocks as a possible explanation for the wave attentuation recorded in seismically active areas. These rocks can absorb and scatter seismic waves, and can also funnel waves down into the crust, they says.

Tests of wave attenuation could serve as a diagnostic tool "to identify the zones that are prone to generating earthquakes," Hamilton and Mooney suggest. This is particularly important west of the Rockies, Where seismologists remain unsure which fault zones can cause large earthquakes. Some faults may appear dormant for hundreds of years, then spring to action.

Attenuation tests might also reveal unknown faults buried beneath thick layers of surface rock and help researchers evaluate the threat of known faults in recently settled areas with limited historical records of seismic activity, Mooney says.

Seismologist Keiiti Aki told SCIENCE NEWS he's very excited by the recent results, which confirm some of his previous research findings. Aki, of the University of Southern California in Los Angeles, has observed that seismic waves from distant earthquakes tend to "ring" for a long time in stable, inactive regions, whereas the ringing dies down much more quickly in seismically active areas.