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Tremor Terror
February 20, 2001

The massive earthquake that shook the state of Gujarat, India last month killed tens of thousands of people and left many more injured. Villages and towns were destroyed by the quake and its aftermath. While earthquakes of this magnitude (7.7 on the Richter scale) are not unheard of, what is troubling about this one is how it happened.

Powerful earthquakes typically happen when the slabs or plates that form the earth’s crust rub together where they meet. But the one in India was different. "Earthquakes can occur within the plates themselves, meaning that the plates can crackle and bend and break away from the plate boundaries," says Arthur Lerner-Lam, associate director of geology and geophysics at Columbia University’s Lamont-Doherty Earth Observatory.

Major earthquakes usually only happen at plate boundaries. For example, the Pacific "Ring of Fire," where several plates meet, is a hot bed of seismic and volcanic activity. At the San Andreas fault line in California, the place where two plates meet is actually visible. It’s thought that the Earth releases its stress—caused by pressure from the hot, flowing mantle beneath it—along the fault line when the plates collide.

How big was it?

In 1935, Charles Richter invented a new way to measure ground movement during earthquake by converting the amplitude of the lines recorded by seismographs into an easy-to-report number. On the Richter scale, each whole number represents a tenfold increase in amplitude. While earthquakes with amplitudes ranging from 1 to 3 can be detected by seismographs, they’re generally not felt. Only earthquakes measuring 5 or more on the Richter scale cause damage. Once they hit 7, they’re considered major, but earthquakes 9 or over are rare.

But the Indian earthquake is a sober reminder that massive quakes can occur anywhere along a plate. That’s what happened in New Madrid, Missouri, in 1811 and 1812. Although this was before the Richter scale was invented, the quakes are believed to have had magnitudes of 8.0 or higher. Charleston, South Carolina also had a large intra-plate quake in 1886.

Such quakes may be due to stress being distributed throughout the plates. "But that violates our original hypothesis that the plates are rigid," says Lerner-Lam. "So we begin to get a new model of the plates—there’s new thinking going into how these plates are constructed, how they behave. We’re beginning to think that they are more dynamic, they’re not as rigid as the models would predict, that in fact they have old zones of weakness that might break under new stress."

Geologists have mapped the plate boundaries along the earth’s surface, but there are places known as blind thrusts, where the fault line isn’t visible on the surface. "The more observations we make—the more carefully we map not just the surface of the crust, but its depths below—the more we find there are intricacies in the way stress is relieved and the way earthquakes are generated that we didn’t have any knowledge of before," says Lerner-Lam.

Minimizing the damage

Cities located near visible fault lines that have a history of earthquakes, like San Francisco, have taken steps to design quake-proof buildings. "We’ve made remarkable advances in building construction throughout the world," says Lerner-Lam. "It goes beyond simple bracing of the structure into what are called active and passive systems that actually help the building respond in some way to the earthquake to minimize the damage."

The ring of fire, an area known for its volcanoes and earthquakes, is shown in yellow.
image: NASA
But if the earth’s crust can shift in unpredictable ways, areas previously thought to be relatively safe in terms of earthquakes may need to worry about large ones. Some cities are already rethinking their potential for earthquake damage. "Particularly in New York City, through the efforts of seismologists and engineers and public officials, this administration has actually put together a building code that includes for new structures some issues associated with earthquake potential," says Lerner-Lam, but that overall, "not a lot has been done in the eastern U.S."

Earthquakes in major urban centers could have particularly disastrous consequences because of their large populations. So even though the probability of an earthquake in a city like New York is low, the potential impact is extraordinary, Lerner-Lam points out.

Although nearly 80,000 earthquakes occur each year, most go unnoticed because they occur in remote areas, including under the oceans. But experts fear that in the absence of serious awareness and research, major within-plate quakes could cause damage to cities such as New York, St. Louis or Chicago.

Elsewhere on the Web

15 largest earthquakes in the U.S.

Real-time views of selected seismograms

Southern California Earthquake Center

Understanding plate motions

Earthquake FAQ

Earthquake Hazards Program—Northern California

by Jill Max

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