Something Weird Is Happening Beneath California’s Deadliest Faults

September 20, 2018

The detection of strange, unpredicted behavior deep below the surface near the San Andreas and San Jacinto faults suggests scientists have an incomplete understanding of the processes responsible for earthquakes in the region.

Over the past four decades, geoscientists have recorded thousands of small earthquakes in California’s San Bernardino basin near the San Andreas and San Jacinto faults. New research published in Geophysical Research Letters suggests many of these quakes, some of which occur at depths between 6 and 12 miles (10 and 20 kilometers), are exhibiting surprising deformation patterns. Instead of slipping in a horizontal manner, many of these earthquakes show vertical movement far below the surface.

This previously undetected movement, dubbed “deep creep,” suggests things aren’t happening beneath the San Andreas and San Jacinto faults in the ways we think they are. And that’s a problem, given the history of the region. Regions along these major faults are vulnerable to damaging earthquakes, the most recent being the 1989 Loma Prieta quake (in the Santa Cruz Mountains south of the Bay area), which registered a magnitude 6.9, killed over 60 people, and caused upward of $6 billion in damages.

Surprisingly, one-third of all small earthquakes detected in the region since 1982 are of the deep creep variety, according to the authors of the new study, geoscientists Michele Cooke and Jennifer Beyer from the University of Massachusetts Amherst. These enigmatic slips are mostly occurring to the northeast of the San Jacinto Fault, and primarily below depths of 6 miles. This strange behavior, the authors say, was completely missed by scientists, the impacts of which now need to be determined.

“These little earthquakes are a really rich data set to work with, and going forward if we pay more attention than we have in the past to the details they are telling us, we can learn more about active fault behavior that will help us better understand the loading [the buildup of seismic pressure] that leads up to large damaging earthquakes,” said Cooke in a statement.

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