An Unexpected ‘Deep Creep’ Found Near San Andreas, San Jacinto Faults

UMass Amherst analysis shows enigmatic pattern in San Bernardino basin is not typical. (Image: UMass Amherst/Michele Cooke)
UMass Amherst analysis shows enigmatic pattern in San Bernardino basin is not typical. (Image: UMass Amherst/Michele Cooke)

A new analysis of thousands of very small earthquakes that have occurred in the San Bernardino basin near the San Andreas and San Jacinto faults suggests that the unusual deformation of some — they move in a different way than expected — may be due to “deep creep” 10 km below the Earth’s surface, say geoscientists at the University of Massachusetts Amherst.

A cartoon showing how anomalous earthquakes near the San Andreas and San Jacinto faults in southern California work in regions deep below the Earth's surface. A UMass Amherst analysis shows this enigmatic pattern in the San Bernardino basin is not typical, and the authors encourage geoscientists to account for them in assessing future fault loading. Image: UMass Amherst/Michele Cooke

A cartoon showing how anomalous earthquakes near the San Andreas and San Jacinto faults in southern California work in regions deep below the Earth’s surface. A UMass Amherst analysis shows this enigmatic pattern in the San Bernardino basin is not typical, and the authors encourage geoscientists to account for them in assessing future fault loading. (Image: UMass Amherst/Michele Cooke)

The new understanding should support more refined assessments of fault loading and earthquake rupture risk in the region, they add. Writing in the current online Geophysical Research Letters, doctoral student Jennifer Beyer and her advisor, geosciences professor Michele Cooke, say the enigmatic behavior is seen in about one-third of the hundreds of tiny quakes recorded during the lull between big damaging quakes, and their possible significance had not been appreciated until now. Cooke said:

Over the past 36 years, the authors point out, seismic stations have recorded the style of deformation for thousands of small earthquakes in California’s San Bernardino basin. They state:

Cooke explains that the usual type of fault in the region is called a strike-slip fault, where the motion is one of blocks sliding past each other. The less common kind, with “anomalous slip-sense,” is an extending fault, where the motion between blocks is like a wave pulling away from the beach, one block dropping at an angle away from the other, “extending” the fault.

This is an area where Cooke, an expert in 3D fault modeling, has done research of her own and where she is familiar with the broader research field, so she decided to try to model what is happening. She began with a hypothesis based on her earlier 3D modeling in the area that had replicated long-term deformation over thousands of years. She said:

As noted, this work has implications for assessing fault loading, Beyer and Cooke point out. Until now, seismologists have assumed that faults in the region are locked — no creep is taking place — and they use data from all the little earthquakes to infer loading on the primary faults.

Cooke adds:

Provided by: University of Massachusetts Amherst [Note: Materials may be edited for content and length.]

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