The same soft powder used to prevent painful chafing between the thighs of marathoners, on babies’ bottoms and in new hiking boots may also be easing friction along the San Andreas fault.
Geologists at the U.S. Geological Survey in Menlo Park discovered talc, the softest known natural mineral, in a two-mile-deep hole drilled into the fault near Parkfield in Central California.
The discovery could explain one of the fault’s most puzzling features — a 100-mile long zone that slowly creeps along rather than sticking for years and then slipping suddenly as other sections of the fault do.
"It’s really one of those questions we would all love to know the answer to, the question of why (some) faults creep and why most faults don’t creep" said geophysicist Roland Burgmann of the University of California, Berkeley, who was not involved in the study. "You wouldn’t believe how many theories there are."
The research by USGS geologists Diane Moore and Michael Rymer, which appears today in the journal Nature, is part of a project called the San Andreas Fault Observatory at Depth, which has been drilling into the fault near Parkfield for four years with the aim of better understanding what causes earthquakes.
The San Andreas fault is the seam between the North American and Pacific tectonic plates. The Pacific plate is moving north relative to North America at a rate of about 2 inches a year. Many smaller faults, such as the Hayward and Calaveras, help accommodate some of that movement, but the San Andreas deals with much of it.
On most sections of the San Andreas fault, the two sides stick together, slowly building stress as the plates move until the friction is overcome and they abruptly jump past each other, causing an earthquake. If a section remains locked long enough, the result can be a major earthquake, such as the 1906 temblor in the Bay Area.
In contrast, the creeping section of the fault between San Juan Bautista and Cholame in Central California moves slowly and steadily, as much as an inch each year. Scientists have struggled to explain why the section behaves so differently.
Some scientists have suggested that the creep could be caused by serpentinite, a relatively soft, greenish rock that was formed in the ocean crust and later thrust onto the continent during a collision with the North American tectonic plate before the San Andreas was formed. But others argue it isn’t soft enough to explain the fault’s behavior.
Moore and Rymer think the more likely culprit is talc that forms along veins and fractures in the serpentinite due to the combination of shearing along the fault and a chemical reaction to hydrothermal water circulating through the rock.
"It’s sort-of this self-forming lubricant," Burgmann said.
Currently, drillers are attempting to extract a 820-foot-long core of rock from the fault zone which will give scientists an even better look at how talc may influence creep.