Earthquakes on Notorious Segment of San Andreas Fault Driven by Deep, Boiling Forces, Study Finds

Earthquakes along a notorious section of the San Andreas Fault are driven by extreme temperatures deep beneath Earth's surface, with rocks being broken up and melted at temperatures of around 350 degrees Celsius, a study has found.

The San Andreas Fault is one of the most dangerous tectonic plates in the world. It separates the North American and Pacific plate, extending around 750 miles through California. Three major cities sit near the fault, potentially putting millions of people at risk if a large earthquake struck.

The Parkfield section of the fault, between Los Angeles and San Francisco, produces moderate earthquakes around magnitude six at relatively regular intervals over the last 150 years. The last earthquake of this size in the area was in 2004. Smaller earthquakes are also known to take place every few months at greater depths.

Because of its regularity, there has been extensive monitoring of the Parkfield section of San Andreas. In 2004, for example, scientists began a project that would see a 2.5 mile hole drilled into the crust, with a massive array of sensors placed within it to provide ongoing monitoring.

In a study published in Science Avances, researchers led by Sylvian Barbot, from the University of Southern California, have now examined the mechanics involved in temblors along the Parkfield section, focusing on the rocks that sit at least 10 miles below the surface.

"Most of California seismicity originates from the first 10 miles of the crust, but some tremors on the San Andreas Fault take place much deeper," Barbot said in a statement. "Why and how this happens is largely unknown. We show that a deep section of the San Andreas Fault breaks frequently and melts the host rocks, generating these anomalous seismic waves."

By looking at 300 years' worth of history at the fault, the team was able to build up a picture of what happens below ground to produce these regular earthquakes. Their simulations showed that after a big earthquake hits, the tectonic plates settle down and move past one another without producing much disturbance above ground. However, this motion eventually leads to the build up of heat, and—at around 350 degrees Celsius (650 degrees Fahrenheit)—the rocks start to melt. When this happens, they slide around more, generating more friction and, as a result, more heat. This eventually leads to rapid movement, which triggers an earthquake.

Their findings, the team say, will help with our understanding of how and when earthquakes may hit. "It's difficult to make predictions, so instead of predicting just earthquakes, we're trying to explain all of the different types of motion seen in the ground," Barbot said.