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Earthquake faults deep in the Earth can glue themselves back together following a seismic event, according to a new study led by researchers at the University of California, Davis. The work, published Nov. 19 in and supported by grants from the National Science Foundation, adds a new factor to our understanding of the behavior of faults that can give rise to major earthquakes. 

“We discovered that deep faults can heal themselves within hours,” said , professor of earth and planetary sciences at �鶹��ý and corresponding author on the paper. “This prompts us to reevaluate fault rheological behavior, and if we have been neglecting something very important.” 

Thomas, �鶹��ý colleague Professor James Watkins and collaborators studied slow slip events or SSEs, a type of seismic movement that is like an earthquake in slow motion. 

Earthquakes occur when the stresses that build up as the Earth’s tectonic plates rub against each other over centuries or millennia are released in a few seconds of violent shaking. 

In about 2002, Thomas said, seismologists identified a new, slower type of seismic event. In a slow slip event, stresses built up over months to years are released in movements of a few centimeters over days, weeks or months. 

Slow slip events in Cascadia fault zone

To characterize deep slow slip events, the team analyzed seismic data from the Cascadia Subduction Zone of the Pacific Northwest, where the Juan de Fuca plate dives under the North American plate. Slow slip events do not behave quite like earthquakes. The same section of fault can re-rupture within hours or days. This suggests both that the fault has at least partly repaired itself and that stresses have been reloaded on a short timescale. 

Small stress changes from the tides show that the fault is reloaded on short timescales, Thomas said. The gravitational forces of the Sun and Moon act on the Earth’s crust just as they act on the oceans to make tides rise and fall. At the same time, the weight of moving sea water also acts on rocks beneath it. 

The missing part of the puzzle is how faults can heal on a short timescale. 

Watkins is a geochemist who studies what happens to minerals under high heat and pressure. His laboratory has equipment to simulate conditions deep in the crust or under a volcano. 

Watkins and Thomas developed experiments where they packed powdered quartz into a silver cylinder, welded it shut and put it under pressure of 1 Gigapascal (10,000 times atmospheric pressure) at 500 degrees Celsius. 

“We’re simulating what happens in the aftermath of a slow slip event,” Watkins said. “We cook it and look at it.” 

They measured how fast soundwaves could move through the “cooked” sample, then opened the cylinders and used electron microscopy to study the structure. 

After compression, the mineral grains were welded together, the researchers found. 

“It’s like quick set fault glue,” Thomas said. “It’s really fast and you can get significant strength recovery.” 

This cohesion — the ability of faults to repair themselves — may be important elsewhere, including in shallower faults and those known for causing major earthquakes.

“Cohesion is neglected in most models,” Thomas said. “Under certain conditions, cohesion may be more important than we thought.” 

Thomas and Watkins recently received a new grant from the National Science Foundation to further study cohesion on earthquake faults. 

“It links events on the microscopic scale to major thrust earthquakes on a scale of hundreds of kilometers,” Watkins said. 

Additional authors on the paper are: Nicholas Beeler, U.S. Geological Survey; Melodie French, Rice University; Whitney Behr, ETH Zürich, Switzerland and Mark Reed, University of Oregon.