Washington: Researchers have identified a mechanism that helps these deeper quakes spread ore than 50 km below surface.
Nearly 25 percent of earthquakes occur more than 50 kilometers below the Earth`s surface, when one tectonic plate slides below another, in a region called the lithosphere.
A team from MIT and Stanford University analyzed seismic data from a region in Colombia with a high concentration of intermediate-depth earthquakes, the researchers identified a "runaway process" in which the sliding of rocks at great depths causes surrounding temperatures to spike.
This influx of heat, in turn, encourages more sliding - a feedback mechanism that propagates through the lithosphere, generating an earthquake.
German Prieto, an assistant professor of geophysics in MIT`s Department of Earth, Atmospheric and Planetary Sciences, said that once thermal runaway starts, the surrounding rocks can heat up and slide more easily, raising the temperature quickly.
He said that they can predict that medium-sized earthquakes, with magnitude 4 to 5, temperature can rise up to 1,000 degrees Centigrade, or about 1,800 degrees Fahrenheit, in a matter of one second.
Scientists have proposed two theories: The first, called dehydration embrittlement, is based on the small amounts of water in rocks` mineral composition. At high pressure and heat, rocks release water, which lubricates surrounding faults, creating fractures that ultimately set off a quake.
The second theory is thermal runaway: Increasing temperatures weaken rocks, promoting slippage that spreads through the lithosphere, further increasing temperatures and causing more rocks to slip, resulting in an earthquake.
Prieto and his colleagues found new evidence in support of the second theory by analyzing seismic data from a region of Colombia that experiences large numbers of intermediate-depth earthquakes - quakes whose epicenters are 50 to 300 kilometers below the surface.
He and his colleagues have published their results in the journal Geophysical Research Letters.