Quake
Caused by Thrust Fault 10 Miles Deep
Upward motion like jolt that hit Northridge
San Francisco Chronicle
David Perlman, Chronicle
Science Editor
Saturday, January 27, 2001
The violent earthquake that struck western India yesterday was born from ponderous but powerful forces deep underground that move entire continents. In many ways, the quake was similar to the deadly ones that have hit Southern California in recent years.
In India, those forces have been building up for more than 40 million years -- shoving the whole subcontinent slowly northward and thrusting the Himalayan mountain chain higher. Occasionally, the tension created by those forces is released in violent quakes like yesterday's.
The ground beneath India and Pakistan is prey to the moving phenomenon called plate tectonics, which generates earthquakes and volcanoes all around the globe.
The Earth's crust is broken into seven huge slabs hundreds of miles thick called tectonic plates that are continuously in motion, floating on Earth's hot, semi-molten mantle beneath.
In California, for example, the Pacific Plate is steadily sliding northward past the North American Plate along the San Andreas Fault Zone, and that motion triggers the Bay Area's major earthquakes.
But India's motion is different. The entire subcontinent forms only part of the Indo-Australian Plate, which butts up against the even larger Eurasian Plate to the north, according to Mary Lou Zoback, chief scientist of the earthquake hazards team at the U.S. Geological Survey's regional headquarters in Menlo Park.
That motion has created the vast Himalayan Plateau, which is constantly being squeezed upward, so that even Mount Everest grows higher year by year. The upthrust of the entire region is continuing now at a rate of about two inches a year, says Frederick J. Ryerson, a geochemist at the Lawrence Livermore National Laboratory, who measured the movement most recently on the ground in Tibet last year.
The magnitude of yesterday's quake was listed by the U.S. Geological Survey as 7.9, although some seismologists, including Zoback, said the quake had a magnitude of 7.5. It is common for the magnitude of a quake to be revised slightly as more data is received from monitoring stations.
Like many other temblors in that vast zone of seismic tumult, the quake erupted suddenly yesterday along a "thrust fault," in which one block of the Earth's crust moved abruptly upward relative to the block on the fault's other side.
The fault itself first ruptured for 20 seconds nearly 10 miles underground, according to the Indian government's seismology division Web site.
Because the quake's epicenter was some 300 miles south of the actual boundary between the two tectonic plates, seismologists are calling it an "intraplate" quake, although it definitely occurred on a thrust fault, the scientists all agreed.
In Southern California, such thrust faults are typical of areas that lie beneath the San Gabriel Mountains.
The Northridge temblor of 1994, for example, struck with a magnitude of 6.7 on a thrust fault, taking a toll of 51 deaths and $44 billion in damage. The San Fernando Valley quake of 1971, with a magnitude of 6.4, killed 65 and did more than $500 million in damage.
The unnamed fault in India ruptured the ground for more than 60 miles. By comparison, only a modest segment of the Sierra Madre Fault broke to cause the Northridge quake.
If that fault ever ruptures along its entire length of more than 50 miles, the death toll could be huge and damage could "run to the trillions," said seismologist Lucile M. Jones at the U. S. Geological Survey regional office in Pasadena.
The India quake struck in a sparsely populated area of a large salt-flat desert, but the Los Angeles region's thrust faults cleave the ground in one of America's most densely populated regions, which makes the hazards far greater, Jones said.
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INTRAPLATE EARTHQUAKES
Earth's crust consists of seven major plates of rock and many smaller ones that
slowly move and interact along seams called plate boundaries. Earthquakes commonly
occur along these boundaries as well as along faults within the plates, called
intraplate earthquakes. An intraplate earthquake is what occurred in India.
A thrust or reverse fault:
When movement occurs along a fault line, compression occurs and the surface above the fault moves up relative to the surface below the fault.
Sources: U.S. Geological Survey National Earthquake Information Center; Incorporated Research Institutions for Seismology
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Major earthquakes that have struck India:
-- April 23, 1999: More than 110 killed in a 6.8-magnitude quake in Chamoli
in the Himalayas in Uttar Pradesh state.
-- May 22, 1997: An earthquake kills 43 and injures 1,000 in Jabalpur in Madhya Pradesh state in central India.
-- Sept. 30, 1993: As many as 10,000 killed and thousands injured in Latur in Maharashtra state in a quake with a magnitude of 6.6.
-- Oct. 19, 1991: Some 2,000 killed in northern India in a magnitude-7 quake.
-- Aug. 20, 1988: More than 1,450 killed in Bihar state in a quake measuring 6.6.
-- Aug. 15, 1950: About 1,538 people killed in the northeastern Assam state in a 8.5-magnitude earthquake. By The Associated Press
E-mail David Perlman at dperlman@sfchronicle.com.
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