Waiting for the Big One: USU Geologist Studies Frictional Behavior of the Southern San Andreas Fault

As Utah State University geologist Alex DiMonte displays a small sample of red clay from the southernmost portion of the San Andreas Fault, it crumbles in her hand, leaving tiny bits of rock and dust on the conference room table.

“This is material known as ‘red clay gouge’ from the exposed San Andreas fault in Mecca Hills at the eastern edge of the Coachella Valley, just north of the Salton Sea,” DiMonte says. “You can see how fragile it is. This fault gouge was less than a mile below the Earth’s surface.”

The doctoral candidate in USU’s Department of Geosciences is lead author on a paper chronicling her detailed study of frictional behavior of the seismically active area published June 25 with faculty mentors Alexis Ault and Srisharan Shreedharan, along with colleague Greg Hirth of Brown University, in the American Geophysical Union journal Geophysical Research Letters. The research was supported by a USU seed grant, the National Science Foundation, the U.S. Geological Survey and the Statewide California Earthquake Center.

The section of the iconic fault DiMonte is studying has not had a major earthquake in more than 300 years, and many think such an event is long overdue.

“I often hear ‘When will the big one hit?’ and there’s not an easy answer,” DiMonte says. “This portion of the San Andreas fault releases some built-up stress through shallow ‘slow slip events’ — that is, slow earthquakes that are spontaneous or triggered by earthquakes hundreds of miles away.”

The distinctive “red clay gouge” is clearly visible as hikers and campers enter the Painted Canyon of Mecca Hills. DiMonte’s new, detailed analyses of this material reveals that, despite the moniker, clay comprises only 30 percent of the gouge’s composition.

“The gouge is rich in quartz and feldspar, but the clay, susceptible to fault movement, is running the show,” says Ault, professor in the Department of Geosciences. “What’s fascinating about this area of the San Andreas fault is that we can see and sample the same fault gouge at the surface that exists at depth, where these slow slip events have occurred, which makes this a special study site.”

DiMonte has taken advantage of this phenomenon to compare field observations with experiments in Shreedharan’s Rock Deformation and Earthquake Mechanics Lab at Utah State, where she replicated the natural conditions of fault slip during her experiments to study the fault gouge samples’ frictional behavior.

Such experiments, she says, show how the fault’s strength or friction changes with the rate of fault slip and if the fault strengthens between sliding events. DiMonte first conducted deformation experiments in labs at Brown University, working with Hirth, and she brought the lessons she learned to her work in Shreedharan’s lab.

“Impressively, Alex was able to extract detailed information about how the San Andreas fault gouge behaves from her lab experiments that she then compared with creep meter and remote sensing observations of recent slow slip events along the fault,” Ault says. “Alex’s field and experimental observations, along with comparisons to geophysical data from recent events, demonstrate how this very gouge can host and promote slow earthquakes. These data could also help reveal how this gouge could localize an earthquake rupture during a future event.”

DiMonte says the study site affords an ideal opportunity to study fault behavior.

“The gouge is massive and full of interconnected slip surfaces all defining the complicated San Andreas fault,” she says. “It’s a seismically active fault with important societal implications. Because of the fault’s recent history — in a geologic sense — the gouge deforming at depth is also at the surface. We have the unique opportunity to observe recent rock displacement throughout the many earthquake cycles, and then erosion also helps rocks that were once at depth come to the surface.”