Scientists Uncover 'Runaway" Mechanism in Earthquakes

Although researchers know that nearly 25 percent of earthquakes occur more than 50 kilometers below Earth's surface, they know very little about these rumblings and what might make them spread. Now, though, scientists have discovered a mechanism that helps deeper quakes like these extend to other areas.

The majority of Earth's seismic activity actually occurs at relatively shallow depths. Over time, abutting plates in the crust build up tension as they shift against each other. This tension ultimately reaches a breaking point and creates a sudden rupture that splinters through the crust. Yet this process isn't feasible for quakes that occur far below the surface.

In order to better understand these deep quakes, researchers analyzed seismic data from a region in Colombia with a high concentration of intermediate-depth earthquakes. These quakes have epicenters that are 50 to 300 kilometers below the surface. This region, known as the Bucaramanga Nest, hosts the highest concentration of intermediate-depth quakes in the world.

So what did they find? The stronger a quake it, the more energy it generates. Yet only two percent of a deeper quake's total energy is felt at the surface. It's likely that the other 98 percent may be released locally as heat, creating an enormous temperature increase that pushes a quake to spread. This provides strong evidence for thermal runaway as a likely cause for intermediate-depth earthquakes. This could be extremely useful for communities in predicting the severity of future quakes.

"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," said German Prieto, one of the researchers, in a news release. "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 have, and what kind of shaking you would expect."

The findings are published in the journal Geophysical Research Letters.