CARRIZO PLAIN NATIONAL MONUMENT, CALIF. — Sitting in the bottom of a 3-foot-deep trench with a hard hat shaped like a cowboy hat, earthquake geologist Lisa Grant Ludwig scanned the bank of tan earth in front of her. Rainstorms had left layer upon layer of sand, silt and pea-sized gravel, going back hundreds of years.
Ludwig was looking for disruptions in the horizontal pattern, evidence that the San Andreas fault had moved in ground-cracking earthquakes.
There it was, a V-shaped fissure that slashed through orderly stripes of sediment. She studied its shape for several silent minutes. Ludwig pressed her fingers against the soil to test the fineness of the grain, as if the history of the San Andreas were written in Braille.
"I've said to people, 'Here's the San Andreas fault,' " she said. "They say, 'Why isn't it more obvious?' That's why we have to spend a lot of time out here."
Ludwig, an associate professor at UC Irvine, has dug nearly two dozen trenches in the Carrizo Plain, a remote saltbush- and tumbleweed-studded grassland about 120 miles northwest of Los Angeles. Her particular spot, the Bidart Fan, is a flat area where streams rolling down the hills nearby spread out and dry up.
She has returned again and again to this place. Every time she comes back with more sophisticated questions, Ludwig said, the Carrizo yields answers. Recently, she and her colleagues have been looking to refine information about the magnitudes of and the time spans between quakes in the last few hundred years, and her most recent data will probably challenge conventional wisdom about the Carrizo.
"This is a scientific gold mine," she said. "You're going to keep working a gold mine."
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Inhabitants of the Carrizo Plain have long recognized its tendency to shake. Early Spanish explorers labeled places where they felt earthquakes with the word "temblor," including the mountain range east of the plain. ("Carrizo" means reed grass.)
A remnant of the vast grasslands that used to cover the San Joaquin Valley, the area is one of the classic places to see disruptions caused by the San Andreas fault.
The landscape clearly shows the warping caused by two pieces of the Earth's crust grinding past each other in a northwest-southeast direction.
Scarce rainfall has meant that few trees or other vegetation obscure the low bluffs and sinkholes alongside the fault. Visitors can easily spot stream channels that run straight down from the mountains and shift abruptly to the right when they cross the San Andreas.
