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A New Fault From Old Data
13 April 2001 7:00 pm
Seismologists have discovered a new fault by analyzing survey data gathered as long as 140 years ago. The newly christened Oldham fault was likely responsible for the devastating 8.1-magnitude Assam earthquake that killed thousands on 12 June 1897, the researchers conclude. The new analysis also explains how a puzzling plateau in northeastern India popped into being.
The deadly quake was thought to have occurred on the Dauki fault line, the junction of two pieces of the Indian tectonic plate, near what's known as the Shillong plateau. Both plates are slowly migrating north, but the weight of the Himalayas slows the leading plate, forcing the following plate to dive under the leader. Friction along the fault locks the plates together until enough strain builds up to cause the plates to slip, unleashing an earthquake. So far, so good.
But it turns out that the Dauki fault was first identified in a six-volume survey of the Shillong plateau, published in 1899 and based on data going back to 1862. The problem is that this survey only covered the southern edge of Shillong. "That left major ambiguities in the interpretation of the data," says seismologist Roger Bilham of the University of Colorado, Boulder. For example, a single fault can't explain how the isolated blocklike Shillong plateau sprouted out of an otherwise smooth plain.
Now, Bilham and Philip England of Oxford University in the United Kingdom have resolved the ambiguities with a combination of modern computation and old-fashioned library sleuthwork. First, Bilham dug up a forgotten 1936 survey of the northern edge of the Shillong plateau. When he combined the 1899 and 1936 surveys, he had data from before and after the quake. "I realized I had all the information I needed to work out all the changes [in the plateau] caused by the Assam quake," Bilham says. Then Bilham and England used thousands of computer models of the Shillong surveys to show that, contrary to what people had thought, the plateau is a wedge of crust pinned between two larger plates. The Assam quake ripped loose when the northern Oldham fault slipped, they report in the 12 April issue of Nature.
The wedge model also naturally explains the origin of the Shillong plateau. "It must have popped up like a watermelon seed squeezed between your fingers" as the plate fragments jammed together, says seismologist James Brune of the University of Nevada, Reno.