The mystery of what caused a great earthquake in northeast India in 1897 that killed several thousand people and reduced all masonry buildings to rubble in a region roughly the size of England finally appears to have been solved.

University of Colorado at Boulder professor Roger Bilham and Oxford University professor Philip England believe the Assam earthquake was most likely caused by two adjacent faults rupturing beneath the Earth's surface in India near Bangladesh. Estimated to be an 8.- magnitude quake, the extremely violent event caused part of the overlying Shillong Plateau to shoot up nearly 15 meters in just three seconds.

"We think we have solved a mystery that has puzzled scientists for over a century," said Bilham. "Before this, nobody really knew what happened."

The new analysis indicates "the acceleration from the epicenter of the earthquake some 16 km 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 appears in today's 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 -- dubbed the Oldham fault -- was under the Shillong Plateau some 160 km south of Bhutan near Bangladesh.

Bilham and England also inferred there must be a second fault on the southern 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."

In the early 20th century, well-known British geologist Richard Oldham concluded that continuing movements on the plateau following the Assam even caused errors in the original data. Oldham -- who went on to discover the core of the Earth -- recommended remeasuring the northern portion of the Shillong plateau.

But the task was not undertaken until his death in 1936. Bilham said the Shillong Plateau uplift in the past 2 million to 5 million years has caused the Indian plate to contract locally by about 38 cm 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."