University of Colorado at Boulder Professor Roger Bilham and Oxford University Professor Philip England believe the Assam earthquake most likely was caused by two adjacent faults rupturing beneath the Earth's surface in India near Bangladesh. Estimated to be an 8.1 magnitude quake, the extremely violent event caused part of the overlying Shillong Plateau to shoot up nearly 50 feet in just three seconds.

"We think we have solved a mystery that has puzzled scientists for over a century," said Bilham, who is a fellow at CU-Boulder's Cooperative Institute for Research in Environmental Sciences. "Before this, nobody really knew what happened."

The new analysis indicates "the acceleration from the epicenter of the earthquake some 10 miles deep exceeded that of gravity, causing boulders, tombstones and even people to be tossed into the air," he said.

"Our findings represent the first quantitative observation of active deformation of a 'pop-up' structure, and confirms that faults bounding such structures can penetrate the whole crust," the researchers wrote. A paper on the subject by Bilham and England will appear in the April 12 issue of Nature.

The blind-thrust Assam earthquake was similar to the earthquake that devastated Bhuj, India, at the opposite end of the continent on Jan. 26, killing an estimated 25,000 people, Bilham said.

Most scientists previously believed the Assam earthquake was caused by a rupture on a Himalayan thrust fault that dipped to the north and propagated south of Bhutan, a small kingdom between India and China.

But historical triangulation measurements made during the Survey of India more than a century ago and new computer models helped Bilham and England conclude the primary fault was under the Shillong Plateau some 100 miles south of Bhutan near Bangladesh.

"We ran a whole series of computer models for different faults until we found one that fit the deformation of the plateau," said Bilham.

The models also show the primary fault, dubbed the Oldham fault, is about 78 miles long, dips away from the Himalayas and lies between five to 20 miles underground. Bilham and England also inferred there must be a second fault on the south edge of the plateau that acts in concert with the Oldham fault to wedge the Shillong Plateau uniformly upward without tilting it.

The Indian plate was being pushed up against the Himalayas, causing a portion of the land surface on the Shillong Plateau to pop up like a segment of a peeled orange would pop up under pressure," he said. "We calculated the slip of the fault to be about 15 meters, one of the largest slips ever calculated for any earthquake."

An expedition led by British Captain J. Bond at the turn of the century discovered 24 feet of uplift on the Shillong Plateau while working for the Survey of India to re-measure triangulation points from an original survey of the plateau in 1862. But his superiors dismissed his results, said Bilham.

In the early 20th century, well-known British geologist Richard Oldham concluded continuing movements on the plateau following the Assam event caused errors in the original data.

Oldham -- who later wrote a large volume describing the Assam earthquake in extraordinary detail and went on to discover the core of the Earth -- recommended re-measuring the northern portion of the Shillong plateau.

But the task was not undertaken until his death in 1936, according to Bilham.

Bilham said the Shillong Plateau uplift in the past two million to five million years has caused the Indian plate to contract locally by about 15 inches a century, reducing the seismic risk to Bhutan but increasing it for Bangladesh, which now has a population of roughly 130 million people.

"Fortunately, an earthquake as powerful as the Assam event only occurs about once every 3,000 years on the Oldham fault," said Bilham. "They are very rare, but could be extremely devastating in this region given the huge population of people now living in Bangladesh and the poor construction practices there."

Related Links
Roger Bilham's Earthquakes and Tectonic Page
University of Colorado-Boulder
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