'Runaway Process' Behind Deep Earthquakes, Study Suggests
Researchers have gained insight into the mechanism that causes deep quakes in the earth to spread and eventually lead to an earthquake. Published in the journal Geophysical Research Letters, the new study suggests the answer lies in a "runaway process," during which the sliding of rocks leads to a jump in temperatures, which triggers even more sliding.
An estimated 25 percent of earthquakes take place more than 30 miles underground, compared to shallower earthquakes, which are much more destructive. But while the mechanics behind shallow quakes are well understood, scientists' concept of what happens in the case of the former is much more vague.
"What we predict is for 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," German Prieto, an assistant professor of geophysics at Massachusetts Institute of Technology, said. "It's a huge amount. You're basically allowing rupture to run away because of this large temperature increase."
Prieto compares the process to heating glass: Try and bend a glass tube at room temperature, and it will shatter. Heat it up, and it becomes malleable. In the case of deep earthquakes, the heat causes the rocks to weaken, giving way to slippage that causes temperatures to rise even more, triggering even more rocks to slip and, in the end, an earthquake.
Prieto and his colleagues uncovered evidence for this theory while examining data from a part of Colombia known as Bucaramanga Nest, home to the highest concentration of intermediate-depth quakes (30-180 miles) in the world.
They analyzed data collected by nearby surface seismometers in order to calculate the total amount of energy each quake released and the amount of energy that made it to the surface in the form seismic waves. Only 2 percent of a deeper quake's total energy was felt at the surface, the researchers found, meaning that the vast majority could very well be released locally as heat.
Knowing this isn't only important for deepening science's understanding of earthquakes, but saving lives, Prieto notes.
"Usually people in Bucaramanga feel a magnitude 4 quake every month or so, and every year they experience a larger one that can shake significantly," Prieto said. "If you're in a region where you have intermediate-depth quakes and you know the size of the region, you can make a prediction of the type of magnitudes of quakes that you can have, and what kind of shaking you would expect."