Earthquakes: The Deadly Side of Geometry
Monastersky, Richard, Science News
In a discovery bound to raise the ire of real estate agents, a seismologist has discovered that living near certain faults poses significantly more danger than previously thought. Other faults, however, may not be as fearsome to their neighbors as they had once seemed.
Earthquakes often appear to strike capriciously, sparing some regions while devastating others nearby. By studying foam rubber models of faults and computer simulations of quakes, James N. Brune of the University of Nevada in Reno found that the geometry of faults may explain some differences among earthquakes. He reported on his research at last month's meeting of the American Geophysical Union in San Francisco.
Brune focused on two types of earthquakes, both stemming from faults that dive into the ground at an angle. In thrust fault earthquakes, land on one side of a fault gets driven up and over land on the other side, much like a block pushed up an inclined ramp. The contrasting situation is a normal fault earthquake, during which rock on one side of the fault gets pulled away from and down relative to the rock on the other side, like a block sliding down a ramp.
According to standard seismological theory, it should make little difference whether a house sits next to a thrust fault or a normal fault. Brune's simulations, however, revealed much stronger shaking near thrust faults, especially on the so-called hanging wall, the land that rises during a quake.
"If this model applies to the real earth, it's very dangerous for people living on the hanging wall of the thrust," says Brune.
The reason stems from differences in the seismic waves generated when a fracture develops and starts to grow. During a thrust earthquake, the fracture sends out a pulse of compression, which squeezes rock. When this pulse reaches Earth's surface, it reflects downward as a pulse of dilatation, which reduces stress on the rock. As these reflected waves intersect the fault, they relieve frictional pressure on the growing quake, allowing the rock on either side of the fault to slide more freely. …