Faults, Slippery When Fast

Slabs of rock that rip apart in an earthquake may accelerate when quartz dust turns into a lubricating gel, says a group of geophysicists. The phenomenon could cause some earthquakes to release more energy and explain a puzzle about the San Andreas fault.

When a geological fault kicks loose, two slabs of rock slide in opposite directions grinding as they go. Many geoscientists think certain faults, such as the San Andreas in California, should radiate heat from the friction between the sliding slabs. But measurements find that the San Andreas isn't as hot as researchers predict. Something seems to be lessening the friction along the fault.

To test the forces at play as rock surfaces slide past each other at various speeds, Terry Tullis, a geophysicist at Brown University in Providence, Rhode Island, and his colleagues ground two pieces of quartz together. They found that at speeds higher than 1 millimeter per second, the amount of friction between the rocks began to drop significantly. Although they couldn't rub the rocks faster than a tenth of a meter per second, the trend of their data suggested that friction would be close to zero between pieces of quartz sliding as fast as they do during an earthquake, the team reports in the 29 January issue of Nature.

Additional experiments revealed that the sliding quartz makes its own lubricant. The team found that dust from the quartz combines with humidity in the air to form a silica gel that lubricates the rock slabs. Like paint, the gel decreases in viscosity the faster it's dragged, thus providing more lubrication the faster the rock moves. Although the increase in slip is dramatic for pure quartz, Tullis and his colleagues couldn't reproduce the effect when they tried the experiment with granite, which contains less silica than quartz.

"It's a tantalizing and unexpected result," says James Rice, a geophysicist at Harvard University, "but not the whole story for earthquake faults." Tullis admits the effect may not be demonstrated in real faults, because silica gel quickly crystallizes into quartz. But if it does occur, Tullis says, the lubrication effect should be incorporated into earthquake models. Less energy dissipated as friction could lead to stronger shaking in an earthquake.

Related site
Tullis's site