Intense Heating At High Pressures Aids In The Spread Of Intermediate-Depth Earthquakes
Almost one quarter of all earthquakes start in the lithosphere, a slice of the Earth about 31 miles below the surface. In a new study published by the journal Geophysical Research Letters, scientists revealed the discovery of a mechanism that helps deep rumblings in the lithosphere spread into larger quakes.
Using seismic data from a region in Colombia known for its lithosphere-generated earthquakes, study researchers from MIT and Stanford University described a "runaway process" involving the sliding of rocks at great depths that generates heat. This heat causes more sliding — a feedback mechanism that eventually results in an earthquake.
According to study author German Prieto, assistant professor of geophysics in MIT's Department of Earth, Atmospheric and Planetary Sciences, once the process begins – the rocks can heat up and slide more easily, resulting in a quick temperature spike.
"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," said Prieto. "It's a huge amount. You're basically allowing rupture to run away because of this large temperature increase."
To reach their conclusion, the researchers analyzed seismic waves recorded by surface seismometers placed in the Bucaramanga Nest region of Columbia. The team was specifically focused on two data parameters: the total energy released by an earthquake, called stress drop, and the energy that is released in the shaking of the ground, called radiated seismic energy.
The study team discovered only 2 percent of a deeper quake's entire energy is detected at the surface. Prieto said the other 98 percent may be released deep in the Earth as heat, which causes a quake to spread. He added this discovery may be useful for predicting the severity of quakes around Bucaramanga.
"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."
Because quakes at these depths do not cause significant damage, they have not received as much attention as shallower quakes. Hiroo Kanamori , a geophysicist at the California Institute of Technology who was not a part of the study team, called for more knowledge around these quakes, as some have caused significant devastation.
"Some intermediate events have caused very strong shaking," Kanamori said. "For example, one of the intermediate-depth aftershocks of the magnitude 9 Tohoku-Oki earthquake in 2011 caused ground motion accelerations as large as, (and) at some locations (even larger than, those) of the main shock. Thus, a better physical understanding of the mechanism of intermediate earthquakes has important implications for hazard mitigation."
Scientists have two different theories on how deeper earthquakes start. A theory called dehydration embrittlement says high pressure and heat at great depths causes rocks to release water, lubricating the surrounding faults and creating fractures that cause a quake.
The thermal runaway theory, which was supported by the new study, posits that rising temperatures weaken rocks and cause slippage that spreads through the lithosphere – eventually resulting in an earthquake.