Earthquakes are pretty straightforward, geologically speaking. Tectonic plates grind against each other at faultlines, building up stress over centuries or millennia. Then that fault slips and the huge amounts of pent up energy is released in the blink of an eye, triggering violent shaking. Slow slip events, or SSEs, are a little more complex. In these seismological interactions, first identified in 2002, fault lines slow dance, grinding, rupturing, and re-rupturing within hours or days. Rather than one or a few single episodes of fierce shaking, SSEs can play out over months, a series of repeated fault ruptures adding up to only a few centimeters of movement over that time.
Seismologists have long wondered how SSEs managed to extend out into slow-motion quakes, particularly how faults managed to rupture and then heal to rupture again. Scientists analyzing seismic data from a tectonically active area of the Pacific Northwest have discovered a potential answer—rock at the interface of two plates actually heals under enormous pressure and temperatures. They published their findings in a Science Advances paper today.
The researchers simulated the conditions that exist after an SSE by developing a chamber that could house 1 Gigapascal of pressure (that’s 10,000 times atmospheric pressure). Into that chamber they poured ground up quartz, sealed it, and heated it to more than 900 degrees Fahrenheit. Examining the contents with electron microscopy, they found that the mineral grains had become welded together. “It’s like quick set fault glue,” Amanda Thomas, a geophysicist at the University of California, Davis, and co-author of the paper, said in a statement. “It’s really fast and you can get significant strength recovery.”
Read more: “A New Way to Predict Earthquakes”
While this process of cementing rock back together is likely at play in deep faults like the one they modeled in the Pacific Northwest, it may also play a role in fault behavior at more dramatic earthquake faults, the authors suggest.
“The observations and experimental results we present here provide strong motivation for reconsidering and better characterizing the role of cementation and resulting cohesion in SSEs and earthquakes in general,” they write in the paper.
The more seismologists understand about the behavior of the ground beneath our feet, the better they can help predict when the Earth will next tremble. 
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Lead image: Duncan A Brown / Shutterstock
