"Unfortunately, we have been forced to reach a very undesirable conclusion," said Professor Roger Bilham of CU-Boulder's geological sciences department. "We set out to try and eliminate the possibility of one or more large, overdue earthquakes in the Himalaya occurring very soon, and we have failed.

"We looked for geophysical loopholes that might provide alternatives to such devastating events, including recent, large earthquakes, smaller earthquakes to relieve the underlying pressure or very slow-moving earthquakes," he said. "But none of these scenarios fit."

A paper on the subject by Bilham, CU-Boulder geological sciences Professor Peter Molnar and Vinod Gaur of the Indian Institute for Astrophysics in Bangalore, India, appears in the Aug. 24 issue of Science.

The conclusion, said Bilham and Molnar, who also are fellows at the CU- headquartered Cooperative Institute for Research in Environmental Sciences, or CIRES, is that at least one 8.1 to 8.3 magnitude earthquake and perhaps as many as seven are overdue. The Himalaya face south toward India in an arc about 2,000 kilometers in length.

"In the past decade using satellite technology we have measured India advancing toward Tibet at a rate of two meters per century," he said. "The historic record indicates only two great earthquakes in the Himalaya in the past two centuries, suggesting that the slip along 70 percent of the arc potentially may exceed four meters," he said.

Looking at the data prior to 1800, the researchers found very few giant historical earthquakes. "In some parts of the Himalaya there may have been no great earthquakes for 500 years, yet they are known to have occurred over time since their effects can be viewed in trenches across the faults that lie beneath the frontal ranges of the Himalaya," he said.

Dividing the Himalaya into 10 regions of about 220 kilometers -- each roughly corresponding to a past great earthquake -- the team found that 70 percent of the arc could have a magnitude 8.1 earthquake and 40 percent could have one as large as 8.3, he said.

The data indicate that the slip zone located about 12 kilometers underground between the Indian and Asian plates is comprised of hot, steamlike fluid. The temperature, pressure and amount of fluids affect the entire seismic system, said Bilham.

"The main driving engine in the system is the movement of the Indian plate, which winds up the Greater Himalaya like a giant spring compressed against the Himalayan Plateau," he said. "Deep beneath Tibet, India slides northward with comparative ease.

"We know the inevitable outcome," he said. "The lock holding the spring will break, propelling the Himalaya southward in a giant earthquake. A giant earthquake is the only solution to have these plates unzip and slide."

Less than one-third of the volatile Himalayan Mountains have slipped in the past 200 years, said Bilham. After calculating a slip rate of 20 millimeters a year along much of the Himalayan arc, six of the 10 regions show a slip rate of from 4 to 8 meters each 200 years -- equivalent to movement that can trigger earthquakes magnitude 8 or above.

"Sadly, to have the Indian and Himalayan plates 'unzip' to remove the geologic stress requires one or more giant earthquakes to occur," said Bilham. "This is where we tried to prove ourselves and the geophysics wrong. We failed."

A large earthquake would cause devastating seismic shaking in the Ganges Plain in India, for example, where the urban population has increased 10-fold in the past century, he said.

The staggering growth of India -- the population has quadrupled since the turn of the century and doubled to 1 billion people since 1950 -- puts an enormous number of people at risk, said Bilham.

"Now we are talking about 10 million people at risk from a single earthquake. Never before have we seen such a huge human geological target."

Dennis Mileti, project director of CU-Boulder's Natural Hazards Research and Applications Information Center, said the potential for great earthquakes in developing countries like India require teams of international experts to advise policymakers on geology, social psychology and mitigation engineering in order to reduce loss of life.

Relatively simple remedies can be effective, Mileti said. For example, people in such earthquake zones should be aware of the need to use rags to cover their faces and prevent suffocation.

In addition, severe injuries to earthquake victims can cause them to die because crushed muscles release deadly toxins into their bodies, he said. If kidney dialysis machines and portable generators were available in such disasters, many more lives could be saved.

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