Deep underground forces explain quakes on San Andreas Fault

September 04, 2020

Rock-melting forces occurring much deeper in the Earth than previously understood appear to drive tremors along a notorious segment of California's San Andreas Fault, according to new USC research that helps explain how quakes happen.

The study from the emergent field of earthquake physics looks at temblor mechanics from the bottom up, rather than from the top down, with a focus on underground rocks, friction and fluids. On the segment of the San Andreas Fault near Parkfield, Calif., underground excitations -- beyond the depths where quakes are typically monitored -- lead to instability that ruptures in a quake.

"Most of California seismicity originates from the first 10 miles of the crust, but some tremors on the San Andreas Fault take place much deeper," said Sylvain Barbot, assistant professor of Earth sciences at the USC Dornsife College of Letters, Arts and Sciences. "Why and how this happens is largely unknown. We show that a deep section of the San Andreas Fault breaks frequently and melts the host rocks, generating these anomalous seismic waves." The newly published study appears in Science Advances. Barbot, the corresponding author, collaborated with Lifeng Wang of the China Earthquake Administration in China.

The findings are significant because they help advance the long-term goal of understanding how and where earthquakes are likely to occur, along with the forces that trigger temblors. Better scientific understanding helps inform building codes, public policy and emergency preparedness in quake-ridden areas like California. The findings may also be important in engineering applications where the temperature of rocks is changed rapidly, such as by hydraulic fracturing.

Parkfield was chosen because it is one of the most intensively monitored epicenters in the world. The San Andreas Fault slices past the town, and it's regularly ruptured with significant quakes. Quakes of magnitude 6 have shaken the Parkfield section of the fault at fairly regular intervals in 1857, 1881, 1901, 1922, 1934, 1966 and 2004, according to the U.S. Geological Survey. At greater depths, smaller temblors occur every few months. So what's happening deep in the Earth to explain the rapid quake recurrence?

Using mathematical models and laboratory experiments with rocks, the scientists conducted simulations based on evidence gathered from the section of the San Andreas Fault extending up to 36 miles north of -- and 16 miles beneath -- Parkfield. They simulated the dynamics of fault activity in the deep Earth spanning 300 years to study a wide range of rupture sizes and behaviors.

The researchers observed that, after a big quake ends, the tectonic plates that meet at the fault boundary settle into a go-along, get-along phase. For a spell, they glide past each other, a slow slip that causes little disturbance to the surface.

But this harmony belies trouble brewing. Gradually, motion across chunks of granite and quartz, the Earth's bedrock, generates heat due to friction. As the heat intensifies, the blocks of rock begin to change. When friction pushes temperatures above 650 degrees Fahrenheit, the rock blocks grow less solid and more fluid-like. They start to slide more, generating more friction, more heat and more fluids until they slip past each other rapidly -- triggering an earthquake.

"Just like rubbing our hands together in cold weather to heat them up, faults heat up when they slide. The fault movements can be caused by large changes in temperature," Barbot said. "This can create a positive feedback that makes them slide even faster, eventually generating an earthquake."

It's a different way of looking at the San Andreas Fault. Scientists typically focus on movement in the top of Earth's crust, anticipating that its motion in turn rejiggers the rocks deep below. For this study, the scientists looked at the problem from the bottom up.

"It's difficult to make predictions," Barbot added, "so instead of predicting just earthquakes, we're trying to explain all of the different types of motion seen in the ground."

The study was supported by grants from the National Natural Science Foundation of China (NSFC-41674067 and NSFC-U1839211) and the U.S. National Science Foundation (EAR-1848192).
USC professor Sylvian Barbot bio:

USC Dornsife College:

USGS Parkfield project:

University of Southern California

Related San Andreas Fault Articles from Brightsurf:

New fault zone measurements could help us to understand subduction earthquake
University of Tsukuba researchers have conducted detailed structural analyses of a fault zone in central Japan to identify the specific conditions that lead to devastating earthquake.

Ancient lake contributed to past San Andreas fault ruptures
he San Andreas fault, which runs along the western coast of North America and crosses dense population centers like Los Angeles, California, is one of the most-studied faults in North America because of its significant hazard risk.

Deep underground forces explain quakes on San Andreas Fault
Rock-melting forces occurring much deeper in the Earth than previously understood drive tremors along a segment of the San Andreas Fault near Parkfield, Calif., new USC research shows.

Signs of 1906 earthquake revealed in mapping of offshore northern San Andreas Fault
A new high-resolution map of a poorly known section of the northern San Andreas Fault reveals signs of the 1906 San Francisco earthquake, and may hold some clues as to how the fault could rupture in the future, according to a new study published in the Bulletin of the Seismological Society of America.

Geoscientists find unexpected 'deep creep' near San Andreas, San Jacinto faults
A new analysis of thousands of very small earthquakes in the San Bernardino basin suggests that the unusual deformation of some may be due to 'deep creep' 10 km below the Earth's surface, say geoscientists at UMass Amherst.

USU geologists detail likely site of San Andreas Fault's next major quake
Utah State University geologist Susanne Jänecke and colleagues identify the San Andreas Fault's 'Durmid Ladder' structure, a a nearly 15.5-mile-long, sheared zone with two, nearly parallel master faults and hundreds of smaller, rung-like cross faults that could be the site of the region's next major earthquake.

Site of the next major earthquake on the San Andreas Fault?
Many researchers hypothesize that the southern tip of the 1300-km-long San Andreas fault zone (SAFZ) could be the nucleation site of the next major earthquake on the fault, yet geoscientists cannot evaluate this hazard until the location and geometry of the fault zone is documented.

'Slow earthquakes' on San Andreas Fault increase risk of large quakes, say ASU scientists
A detailed study of the California fault has discovered a new kind of movement that isn't accounted for in earthquake forecasting.

Parkfield segment of San Andreas fault may host occasional large earthquakes
Although magnitude 6 earthquakes occur about every 25 years along the Parkfield Segment of the San Andreas Fault, geophysical data suggest that the seismic slip induced by those magnitude 6 earthquakes alone does not match the long-term slip rates on this part of the San Andreas fault, researchers report November 28 in the Bulletin of the Seismological Society of America (BSSA).

Fault system off San Diego, Orange, Los Angeles counties could produce magnitude 7.3 quake
The Newport-Inglewood and Rose Canyon faults had been considered separate systems but a new study shows that they are actually one continuous fault system running from San Diego Bay to Seal Beach in Orange County, then on land through the Los Angeles basin.

Read More: San Andreas Fault News and San Andreas Fault Current Events is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to