Expanding “Hot Spots” May Trigger Earthquake Fault Slips

Earthquake fault failure is complex. The stressed rock weakens and leaps in a multistage pattern: the rock’s shear strength drops suddenly, recovers, and then drops again. The last stage is already fairly well understood. To over-simplify: pressure, friction, and ambient heat melt the rock, and the resulting viscous mess lubricates the fault.

Until now, though, geologists haven’t been quite sure how the fault loses its grip the first time. In the 30 August Nature, two Scripps Institute of Oceanography geophysicists attribute the first slip to small, localized “hot spots” of expanding material that force the fault faces apart. This reduces the pressure on the rest of the rock face, reducing the friction between them and lowering the resistance to slip.

Kevin M. Brown and Yuri Fialko reconstructed an industrial lathe to reproduce geological pressures and slip speeds in an 80-mm-diameter ring of gabbro (a coarse, black, igneous rock similar to basalt). The rotating ring pushed against a stationary rock plate at pressures up to 5 megapascal, slip velocities of 0.05 to 0.25 meters per second, and temperatures of 250 C to 550 C (well below the melting point of uncompressed stationary rock).  

This may be the first time such a lathe has been adapted to high-speed friction experiments. Instrumentation included a set of thermocouples set into the rock plate in the flywheel, three linear variable differential tranformers to measure rock wear or possible expansion (dilatancy) of material in the interface, and a load cell for calculating torque. Building the rig required adding a massive flywheel, reinforcement, and stiffening to reduce vibration and tool-face chatter to geological levels. Nonetheless, the authors report, “The onset of weakening in our experiments is manifested in a transition to regular, high-frequency oscillations…, and sometimes a barely audible rumble.” 

At a given temperature and pressure, the researchers found a critical velocity—the slip speed at which the shear strength would suddenly plummet and the sample would jump. As the sample neared this critical velocity, an increment  as small as 0.01 meter per second could trigger the transition.

While frictional “flash melting” of microscopic asperities (the peaks of rough material) might possibly cause the sudden failure, the researchers think the evidence favors another mechanism: “positive feedback between heating and dilatancy.”

Tiny particles gouged and crumbled from the sliding walls of rock fill the fault face. Imagine that you are pressing a layer of irregular gravel between your flat, somewhat sticky palms as you try to slide them against one another. You’ll find that the stones twist, roll, and tumble, thickening the gravel layer and forcing your palms apart as you press them together harder. That’s dilatancy. If this happens in a geologic fault, the pressure on the dilated material increases tremendously. The thickening layer heats up even as it pushes the two faces of the fault apart. This both increases the local pressure and unloads the surrounding regions of rock. The total frictional resistance drops abruptly and the dilated material melts and smears as the fault jumps, leaving a trail of “melt welt.”

And In Other Earthquake News…

As the Scripps paper arrived from La Jolla, Calif.., an unusual swarm of small earthquakes seemed finally  (one hopes) to be fading from the area around Brawley, Calif., just over 120 miles east-by-north along Interstate 8.

From 25 through 28 August, seismologists logged more than 400 tremors of magnitude 1.0 or greater in the region, according to the LA Times. The US Geological Survey Earthquake Hazards map shows 144 quakes (magnitude 2.5 to 5.5) for the 23-28 August period—the great majority of them coming on the 26^th and 27^th.

One tool in the seismologist’s arsenal is USArray, a relatively new network of four seismic observatories: the Reference Network (about 100 permanent stations covering the country in a 300-km grid), the Flexible Array (more than 2,000 telemetered seismographs available for loan and short-term placement), the Magnetotelluric Array (deployed for electromagnetic field measurements), and the Transportable Array. (If you want to see the earth move in your area, you can monitor your local seismic station at http://usarray.seis.sc.edu/.)

The Transportable Array is a migrating 500-mile-wide band of 400 high-quality, portable seismographs that covers the country in a 42-mile grid from Canada to Mexico and the Gulf. The portable array started with NSF funding in 2007, beginning on the Pacific coast. Today, it has rolled across the country to the eastern edge of the Midwest, reaching from Michigan’s Upper Peninsula to Louisiana and from Ohio to Florida.

From November 2009 to September 2011, the Transportable Array covered Texas. University of Texas geophysicist Cliff Frohlich analyzed the data, paying particular attention to the state’s oil-rich Barnett Shale. After analyzing more than 1300 events and discarding quarry blasts and signals straying in from out-of-state, he identified 149 earthquakes in the region—nothing like the 400 logged last week in Brawley, but many times the 8 listed by the National Earthquake Information Center. In a recent analysis published in the Proceedings of the National Academy of Science, Frohlich persuasively demonstrated that the quakes cluster around the state’s highest-volume injection wells, those pumping 150,000 barrels of water per month (BWPM) or more into the ground.

His conclusion: “The most significant result of this investigation is that all of the better located epicenters were situated within a few kilometers of one or more injection wells…. It is possible that some of these earthquakes have a natural origin, but it is implausible that all are natural.”

Images: Scripps Oceanographic Institute, USArray