`Runaway` heat behind deep earthquakes

Washington: Researchers have gained insight into the mechanism that causes deep quakes in the Earth to spread and eventually lead to an earthquake.

Nearly 25 per cent of earthquakes occur more than 50 kilometres below the Earth`s surface, when one tectonic plate slides below another, in a region called the lithosphere. Scientists have thought that these rumblings from the deep arise from a different process than shallower, more destructive quakes. But limited seismic data, and difficulty in reproducing these quakes in the laboratory, have combined to prevent researchers from pinpointing the cause of intermediate and deep earthquakes. A team from Massachusetts Institute of Technology (MIT) and Stanford University has identified a mechanism that helps these deeper quakes spread. By analysing 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. Researcher German Prieto said that once thermal runaway starts, the surrounding rocks can heat up and slide more easily, raising the temperature very quickly. "What we predict is for medium-sized earthquakes, with magnitude 4 to 5, temperature can rise up to 1,000 degrees Centigrade, in a matter of one second," Prieto said. "It`s a huge amount. You`re basically allowing rupture to run away because of this large temperature increase," said Prieto. Prieto said that understanding deeper earthquakes may help local communities anticipate how much shaking they may experience, given the seismic history of their regions. The majority of Earth`s seismic activity occurs at relatively shallow depths, and the mechanics of such quakes is well understood: Over time, abutting plates in the crust build up tension as they shift against each other. This tension ultimately reaches a breaking point, creating a sudden rupture that splinters through the crust. However, scientists have determined that this process is not feasible for quakes that occur far below the surface. Essentially, higher temperatures and pressures at these depths would make rocks behave differently than they would closer to the surface, gliding past rather than breaking against each other, researchers said. The study was published in the journal Geophysical Research Letters.