BROWN U. (US) — In an earthquake, rock surfaces sliding past each other create intense stress and heat—but only in super-small places where the surfaces actually touch.
This intense heating can occur even while the temperature of the rest of the fault remains largely unaffected, a phenomenon called flash heating.
Most earthquakes that are seen, heard, and felt around the world are caused by fast slip on faults. While the earthquake rupture itself can travel on a fault as fast as the speed of sound or better, the fault surfaces behind the rupture are sliding against each other at about a meter per second.
Computer-simulated topography shows high points— asperities (in red)—on the rock surface. When in contact with asperties on the adjacent surface, these asperities may undergo intense flash heating in an earthquake. (Credit: Mark Robbins and Sangil Hyun, Johns Hopkins University)
The mechanics that underlie fast slip during earthquakes have eluded scientists, because the conditions are difficult to replicate in the laboratory.
“We still largely don’t understand what is going at earthquake slip speeds,” says David Goldsby, associate professor of geological sciences (research) at Brown University, “because it’s difficult to do experiments at these speeds.”
In experiments mimicking earthquake slip rates, Goldsby and Brown geophysicist Terry Tullis show how tiny contact points support all the force across the fault and when two fault surfaces slide against other at fast slip rates, scattered bumps called asperities may reach temperatures in excess of 2,700 degrees Fahrenheit, lowering their friction.
“This study could explain a lot of the questions about the mechanics of the San Andreas Fault and other earthquakes,” says Tullis, professor emeritus of geological sciences.
The experiments simulated earthquake speeds of close to half a meter per second. The rock surfaces touched only at the asperities, each with a surface area of less than 10 microns—a tiny fraction of the total surface area.
The experiments, reported in Science, showed that when the surfaces move against each other at high slip rates, heat is generated so quickly at the contacts that temperatures can spike enough to melt most rock types associated with earthquakes.
Yet because the intense heat is confined to the contact flashpoints, the temperature of the surrounding rock remains largely unaffected by the microscopic hot spots, maintaining a “room temperature” of around 77 degrees Fahrenheit.
“You’re dumping in heat extremely quickly into the contacts at high slip rates, and there’s simply no time for the heat to get away, which causes the dramatic spike in temperature and decrease in friction.
“The friction stays low so long as the slip rate remains fast. As slip slows, the friction immediately increases. It doesn’t take a long time for the fault to restrengthen after you weaken it. The reason is the population of asperities is short-lived and continually being renewed, and therefore at any given slip rate, the asperities have a temperature and therefore friction appropriate for that slip rate. As the slip rate decreases, there is more time for heat to diffuse away from the asperities, and they therefore have lower temperature and higher friction.”
Flash heating and other weakening processes that lead to low friction during earthquakes may explain the lack of significant measured heat flows along some active faults like the San Andreas Fault, which might be expected if friction was high on faults during earthquakes.
Flash heating in particular may also explain how faults rupture as “slip pulses,” wrinkle-like zones of slip on faults, which would also decrease the amount of heat generated.
If that is the case, then many earthquakes have been misunderstood as high-friction events.
“It’s a new view with low dynamic friction. How can it be compatible with what we know?” asks Tullis, who chairs the National Earthquake Prediction Evaluation Council, an advisory body for the U.S. Geological Survey. “Flash heating may explain it,” Goldsby says.
The U.S. Geological Survey funded the research.
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