Geologists dig for earthquake history
CARRIZO PLAIN NATIONAL MONUMENT, Calif. - Sitting in the bottom of a 3-foot-deep trench wearing 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. 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 the University of California, Irvine, has dug nearly two dozen trenches in the Carrizo Plain, a grassland about 120 miles northwest of Los Angeles. Her particular spot, the Bidart Fan, is a flat area where streams rolling down the nearby hills spread out and dry up.
She has returned again and again to this place. Every time she arrives with more sophisticated questions, Ludwig said, the Carrizo yields answers. Recently, she and her colleagues have been looking to refine information about the magnitude of quakes and the time spans between them in the past 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.’’
Ludwig said her group is confident that they now have accurate dates for at least six earthquakes in the past 800 years. The trenches also show a shorter average interval between quakes, about 100 years.
David Schwartz, a US Geological Survey scientist who is not involved in the group’s study but has been observing its work, said the results probably would lead to a “fundamental revision of how that entire part of the San Andreas is moving.’’
It also probably will change the Uniform California Earthquake Rupture Forecast, which estimates probabilities for earthquakes in California, he said.
“It’s been 150 years since the most recent earthquake - the one in 1857 - on the Carrizo and, even though there’s a lot of uncertainty, you could view us as being much closer to reaching the time when it might slip again,’’ he said. 