‘Deep Creep’ Movement Near San Andreas, San Jacinto Faults Explains Unusual California Earthquakes

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Geoscientists who analyzed thousands of small earthquakes that occurred near the San Bernardino basin near California’s San Andreas and San Jacinto faults discovered a strange and unexpected type of movement 10 km below the Earth’s surface.

Researchers think that the movement, known as “deep creep,” could be behind the unusual earthquake formations recorded in the region over the past 36 years.

Jennifer Beyer and Michele Cooke, from the University of Massachusetts Amherst, developed a 3D model based on readings from thousands of small quakes in the hope that the new model can help experts better predict when and where the next major earthquake may occur.

California is believed to be overdue for a particularly strong earthquake called the “Big One” which makes better earthquake prediction important.

Data showed that many of the earthquakes deep below have a different style of deformation not similar to other deformations in the region. The ruptures showed vertical movement instead of the expected horizontal slip.

Modeling revealed that the vertical movement can be produced in the basin of the northern part of the San Jacinto fault, which turns out to be moving backward instead of staying locked like the San Andreas fault.

Deep Creep Found Near San Andreas California
Featured image credit: David McNew Getty Images

“This basin was an extension in those models unlike the surrounding regions of strike-slip,” Cooke said in a press release published by the University of Massachusetts Amherst.

“The extension was limited to within the basin just like the pattern of the anomalous extensional earthquakes. That gave me a clue that maybe those faults weren’t locked as they should be between big earthquakes, but that at depths below 10 km, they were creeping.”

Scientists typically use GPS to study creep in faults but this does not work in this case since the San Jacinto and San Andreas faults are too close together. The researchers explained that this is the reason no one had seen the “deep creep” before; the traditional method used to detect it does not work.

In their paper, which was published in the Geophysical Research Letter on Aug. 30, the researchers explained that small quakes occurring to and between the faults may have a different style of deformation compared with large group rupturing earthquakes that are generated along active faults.

What this means, according to Beyer and Cooke, is that scientists should not rely on the information recorded by small earthquakes in the San Bernardino basin to predict the loading of the nearby San Jacinto and San Andreas faults.