Scientists Look for Lessons in India Quake
Earthquake science is a worldwide discipline, and when a large quake occurs in a spot accessible to study, like the Jan. 26 temblor in Gujarat state in India, it mobilizes seismologists everywhere.
They want to know what that quake, now assessed at magnitude 7.7, tells us about possibly similar seismic settings much closer to home. What can be learned about quake hazards in, say, California?
Already, quite a few American scientists think there are lessons to be learned here from what happened in India. Some have already drawn parallels to the New Madrid, Mo., quakes of the early 1800s. Others are finding likenesses with fault systems in Southern California--it was a similar thrust fault that was responsible for the 1994 Northridge earthquake.
“This is the earthquake we’ve been waiting for,†said Roger Bilham, professor of geology at the University of Colorado.
It was the second major quake in Gujarat in 182 years. Both were extremely powerful, despite the fact that the rupture zones were comparatively short, little more than 40 miles.
For Sue E. Hough, a seismologist with the U.S. Geological Survey in Pasadena, the implication is that quakes as powerful as the Gujarat temblors could happen in the Los Angeles Basin.
“There’s been other evidence that quakes approaching 7.5 [far more powerful than the 1994 Northridge quake of 6.7] have occurred [in prehistoric times] in the L.A. Basin on the larger thrust faults,†Hough said.
“The predictability, as to precise time, however, is pretty much nil.â€
Thomas Heaton, professor of earthquake engineering at Caltech, nonetheless saw the Gujarat quake as “an example of a worst-case scenario†for Southern California.
“This was a thrust earthquake,†he said. “Certainly, we have large thrust faults in the Los Angeles area, so we are quite interested in hearing about this earthquake.â€
The Gujarat quake was felt more than 1,000 miles from its epicenter, and the damage zone extended 300 miles. Deaths occurred more than 200 miles from the epicenter.
Maps developed for historic American quakes show that the New Madrid, Mo., quakes of the winter of 1811-12--at least three were in the 7 to 8 magnitude range--were felt strongly all over the Eastern United States, from Boston to Nebraska, north to Canada and out into the Gulf of Mexico.
By contrast, California quakes, occurring in looser, less stable rocks, don’t seem to be felt so far away. The hazard zone for specific jolts is thus smaller.
The Gujarat earthquake was comparable in this sense to New Madrid.
What is involved is attenuation, or the rate of dilution or lessening in severity.
Hiroo Kanamori, former director of the Caltech Seismological Laboratory, suggested that the study of the shaking pattern outward from Gujarat could result in the development of a kind of attenuation scale to determine how wide an area would be shaken in quakes occurring in various kinds of Earth’s crust.
The Indian quake, said Bilham, “is going to calibrate two things we know little about. One is attenuation.â€
The other, he said, may be how rocks in stable crust behave at depths as great as 18 miles. He categorized the Indian earthquake, and the ones at New Madrid, as events taking place between the great tectonic plates where most quakes occur around the world.
One quake has bearing on the others, half a world away, he said. And since powerful Gujarat quakes occurred 182 years apart, that may even be a hint at a repetition soon to be expected of the New Madrid quakes.
If such a recurrence were to take place, it would be devastating for many states in the Midwest. When the New Madrid quakes occurred 190 years ago, most of the affected area was thinly populated. Today, millions of people live there, and very few construction precautions have been taken against quake damage and casualties.
Two Indian researchers, Kusala and C.P. Rajendra, a couple working with Mary Lou Zoback, a senior U.S. Geological Survey scientist, also have postulated Gujarat implications for the New Madrid fault zone.
But not all scientists agree. Comparisons of Gujarat with New Madrid are invalid, insisted Seth Stein, a Northwestern University expert on subduction zones and the intercontinental plates.
Plate theory has been changing in recent years, Stein said in an interview, and many scientists are coming to realize that many plates are not lines, like the San Andreas fault, that can be straddled by a human being. Rather, they are zones that may be 1,000 miles wide.
Accordingly, Stein said, the Gujarat quake was within the boundary zone where the Indian and Eurasian tectonic plates meet, and cannot be considered a mid-continental earthquake like New Madrid.
“I suspect extrapolating from the Gujarat earthquake to the next major New Madrid earthquake is like extrapolating from a motorcycle hit by a truck to a car hitting a tree,†he said. “I’d be very skeptical about any such implications.â€
Some California quake scientists “have taken a look and say the Gujarat quake [like the New Madrid] was . . . not on a plate boundary,†said the Geological Survey’s Hough.
“It’s in a part of the crust that people say would be relatively stable,†she said. “The New Madrid is much more in the center of a continent, but there are similarities in the crust, and the propagation of earthquake waves [the attenuation] too.â€
It’s certainly not new that quake scientists disagree. Quakes such as Gujarat may well provide research opportunities, however, that will help resolve their questions.
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Comparisons, Half a World Away
Although disputed in some quarters, some U.S. earthquake scientists believe that the magnitude 7.7 earthquake Jan. 26 in Gujarat state in India bears comparison to the powerful 1811-12 intracontinental earthquakes in the New Madrid fault zone in the American Midwest. Both thrust quake sequences were felt over vast areas and caused damage far from the epicenters. A similar thrust quake occurred in 1819 in the same part of India, and some scientists speculate that this time frame may portend a recurrence fairly soon of large quakes in the New Madrid fault zone. Also, scientists say the comparative shortness of the quake rupture zone in India, only about 40 miles, demonstrates that a very large quake could occur in the Los Angeles Basin, which also has short but potent thrust faults.
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Source: Sue Hough, U.S. Geological Survey
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Reich can be reached at [email protected]
