Strange Movement Has Been Detected Near San Andreas Fault

The "deep creep" is happening 10 km below the Earth's surface.

Geoscientists have detected unexpected movement in the San Bernardino basin near the San Andreas and San Jacinto faults. The vertical movement, dubbed Deep Creek, is happening 10 km below the Earth’s surface and is likely to be responsible for unusual ruptures in the region.

Researchers have reached this conclusion based on record that includes analysis of thousands of very small earthquakes in the San Bernardino basin. Although small earthquakes are often overlooked by people, researchers say they can lead to more destructive and large earthquakes.

“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 that leads up to large damaging earthquakes.” Geosciences professor Michele Cooke from University of Massachusetts Amherst said.

Southern California's San Bernardino basin has been studied intensely over the years. Data shows that many earthquakes below 10‐km depth have a very different style of deformation that does not match other deformations in the region. Rather than showing expected horizontal slip, many sections are sliding vertically.

To understand the phenomenon, researchers developed crustal deformation models. The modeling showed that vertical movement can be produced in the basin if the nearby northern San Jacinto fault creeps at depth. Also, this portion of the fault is constantly moving rather than locked as it should be between large earthquakes. It implies that small earthquakes that occur near active faults can have very different style of deformation than the large earthquakes.

“The typical way we look for creep is to use GPS stations set up on each side of the fault. Over time, you can note that there is movement; the faults are creeping slowly apart. The problem here is that the San Andreas and the San Jacinto faults are so close together that the GPS is unable to resolve if there is creep or not. That's why no one had seen this before. The traditional way to detect it was not able to do so,” said Cooke. “In this paper we’ve shown that there is a way to have these weird tiny earthquakes all the time next to the San Jacinto Fault below 10 km, which is where deep creep is happing.”

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