Faultline samples could hold clues

Core samples from inside the San Andreas fault may provide scientists with their first clues regarding how earthquakes begin, why they happen when they do and why some are so much stronger than others.

Those samples, drilled just weeks ago from a test site near Paso Robles, were unveiled by Stanford University and U.S. Geological Survey scientists Thursday. The group also announced plans to establish an observatory two miles underground, deep within California’s biggest fault, to measure and study earthquakes as they happen.

“For the first time, scientists can hold a piece of the San Andreas fault in their hands,” said Mark Zoback, a geophysicist with Stanford University.

Until now, researchers had to make do with what they could observe from the earth’s surface. The samples, along with the observatory, may answer many longstanding questions about the fault responsible for the 1906 earthquake and 1989’s Loma Prieta quake.

Geologists installed a steel channel in the fault to drill out core samples.

The samples include rock found all over California — along with serpentine, which comes from the earth’s crust on the ocean floor, Zoback said. That may provide a key clue regarding how active the San Andreas fault is, because serpentine, a relatively weak mineral, produces talc, an even weaker mineral. Talc’s friction-reducing properties may be lubricating the fault line.

“It could be that the San Andreas fault is located where it is because of where the serpentine is,” Zoback said. “And then the serpentine gets caught up and smeared along the length of the fault.”

Geologists chose the test site, in the small town of Parkside, because it has repeating “mini-earthquakes” that are not felt on the earth’s surface but provide a steady stream of data about the fault line’s movement over time, said William Ellsworth, a geologist with the USGS.

The observatory will consist of a 7-inch channel lined with scientific instruments that will study the fault over a 15-year period. Those instruments will detect clues regarding what happens before, during and after each quake and send data to computers on the earth’s surface for analysis.

“We don’t understand the physics that control the start of an earthquake, or why they stop,” Ellsworth said. “We’ve already recorded thousands of earthquakes and we’re beginning to see new physics not observed on the surface.”

bwinegarner@examiner.com

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