Researchers at the University of California, Davis, have found that deep earthquake faults can heal themselves within hours, challenging existing models of seismic activity. This discovery, based on studies of slow slip events in the Cascadia Subduction Zone, suggests that mineral grains weld together under high pressure and heat. The findings could reshape how scientists predict major earthquakes.
A new study published on November 19 in Science Advances reveals that faults deep within the Earth can regain strength much faster than previously thought. Led by Amanda Thomas, a professor of earth and planetary sciences at UC Davis, the research was supported by National Science Foundation grants. The team focused on slow slip events (SSEs), which are gradual releases of built-up stress along tectonic plates, occurring over days, weeks, or months and involving movements of just a few centimeters.
In the Cascadia Subduction Zone, where the Juan de Fuca plate slides beneath the North American plate in the Pacific Northwest, seismic data shows that the same fault segment can slip repeatedly within hours or days. This rapid recurrence indicates quick recovery of fault strength, influenced even by small tidal forces from the Sun, Moon, and shifting seawater.
To investigate, James Watkins, a geochemist at UC Davis, simulated post-SSE conditions in the lab. They subjected powdered quartz to 1 gigapascal of pressure—10,000 times atmospheric pressure—and 500 degrees Celsius. Analysis with electron microscopy revealed that mineral grains had welded together, forming a cohesive bond. "It's like quick set fault glue," Thomas said. "It's really fast and you can get significant strength recovery."
Thomas emphasized that cohesion, the process of faults regaining strength, has been overlooked in most earthquake models. "We discovered that deep faults can heal themselves within hours," she noted. "This prompts us to reevaluate fault rheological behavior."
The study suggests cohesion may play a key role in other tectonic settings, including shallower faults linked to large earthquakes. Watkins added, "It links events on the microscopic scale to major thrust earthquakes on a scale of hundreds of kilometers."
Additional contributors include Nicholas Beeler from the U.S. Geological Survey, Melodie French from Rice University, Whitney Behr from ETH Zürich, and Mark Reed from the University of Oregon. The team has received a new NSF grant to further explore fault cohesion.