Because the Indian and the Eurasian tectonic plates collide in northern India, pushing up the Himalayas, earthquakes there are not uncommon. But the 1897 temblor was unusually large. To unravel the mystery, the team compared data from a century-old survey of India with several new computer models of geological faults. One earlier theory proposed that a rupture on the Himalayan thrust fault, which then propagated south of Bhutan, caused the quake. But the new analysis suggests that the primary faultthe 78-mile-long Oldham Faultwas beneath the Shillong Plateau, some 100 miles south of Bhutan near Bangladesh.
"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," Bilham explains. "We calculated the slip of the fault to be about 15 meters, one of the largest slips ever calculated for any earthquake." The thrust was so strong, in fact, that boulders, tombstones and even people were tossed in the air, he notes.
"Fortunately an earthquake as powerful as the Assam event only occurs about once every 3,000 years on the Oldham Fault," Bilham says. "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."