San Andreas Fault: Unexpected Underground 'Deep Creep' Could Explain Mystery Earthquakes

Thousands of tiny earthquakes have helped expose unusual rumblings taking place beneath the surface of southern California.

Scientists probing California's complex fault system now think many of these small vibrations reveal an ongoing deep creep some 6 miles underground. Seismologists should rethink the way they model the energy building up below the surface in light of his surprising motion, researchers wrote in a study published in Geophysical Research Letters.

The San Andreas and San Jacinto faults slice through California. San Andreas is a major fault, cutting through much of the length of the state. San Jacinto is smaller, running in parallel to a section of San Andreas to the south of the state. The San Bernardino basin lies between the two.

Both San Andreas and San Jacinto are strike-slip faults. This means the two sides of each boundary slide past the other like blocks. But the region also sees a more unusual motion, where a wave-like movement "extends" the fault. "These only occur in this one small area, and nobody knew why," study author and University of Massachusetts Amherst geosciences professor Michele Cooke explained in a statement. "We did the modeling that helps to explain the enigmatic data."

About a third of the hundreds of small quakes that take place between larger events show evidence of this strange movement. Even so, the significance of the little quakes to the larger ones was previously unknown, the researchers wrote.

Cooke used her experience with 3D fault modeling to try and figure out what was happening deep underground. The patterns of extension within the basin "gave me a clue that maybe those faults weren't locked as they should be between big earthquakes," she explained. "At depths below 6 miles, they were creeping."

Scientists normally use GPS tracking to measure faults creeping apart, but San Jacinto and San Andreas, Cooke said, are too close together for the usual methods. Instead, her team's models show this creeping is a "plausible" explanation for many tiny quakes. "The model may not be perfectly correct, but it's consistent with observations."

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File photo: California from the air. California's tectonic activity leaves its mark in the state's devastating landscapes. Getty Images

Seismologists normally treat the faults as locked, so the researchers think the normal calculations of energy building up at these boundaries could be compromised. "Scientists should not use the information recorded by these small earthquakes in the San Bernardino basin to predict loading of the nearby San Andreas and San Jacinto faults," the authors wrote.

Praising the "quite well founded" study, seismologist John Vidale who directs the Southern California Earthquake Center, told Newsweek its results might prove useful to certain specialists, even if they are "sad to see the interpretability of their models undercut." However, he added the study's implications are "profoundly ambiguous," and said "no one serious pitfall already made by modelers is singled out."

Understanding fault loading is critical in California, which has been rocked by incredibly powerful earthquakes in the past. In 1989, for example, a 6.9 magnitude earthquake struck a state park about 10 miles northeast of Santa Cruz. It caused an estimated $6 billion in damage, resulting in 63 deaths and thousands of injuries.

This article has been updated with comment from John Vidale. Michele​ Cooke did not immediately respond to Newsweek's request for comment.