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Unlocking the secrets of California's past earthquakes

Geologist Lisa Grant Ludwig and her colleagues are challenging the conventional wisdom about the San Andreas Fault's path through the Carrizo Plain.

June 21, 2009|Jia-Rui Chong

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."


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.

Human settlement has also been generally sparse, though tribes of Native Americans, then cattle ranchers and dry farmers have inhabited the plain. As a result, the layer cake of sediments underfoot has remained well-preserved.

Since 2001, much of the plain has been protected as a national monument, managed by the federal Bureau of Land Management, the state Department of Fish and Game and the Nature Conservancy.

Scientists started putting the pieces together at the turn of the 20th century, naming the San Andreas fault and linking the crack threading through the Carrizo to the Ft. Tejon earthquake in 1857 and the San Francisco earthquake in 1906.

Previous studies of features here, including the Bidart Fan and, about three miles away, the dry stream bed known as Wallace Creek, showed hundreds of feet of motion on the fault over thousands of years, said J. Ramon Arrowsmith, a geologist at Arizona State University and one of Ludwig's collaborators. But there was little detail on the pacing, or how they happened on a time scale of hundreds of years, so Ludwig's group looked for a place that preserved a more richly layered package of sediments.

Wallace Creek "effectively looks like steady motion," Arrowsmith said. "At Bidart, we can see individual snapshots."

They want to know how the earthquakes moved the plates, he said. "Was it a bunch of little guys and a big one? Was it a bunch of medium-sized ones?"

Ludwig dug her first trench in the Bidart Fan, named after the family that owns the land, about 20 years ago.

In her first trenches, Ludwig saw traces of seven earthquakes. But she had to find chunks of charcoal to date the sediment around the ruptures and could find enough to put time brackets around only five temblors. Those quakes appeared to have taken place between about 1200 and 1857.

The work raised questions about the intervals between earthquakes on the Carrizo because the first four quakes appeared to be clustered, occurring roughly every 100 years. Then there was a gap of about 400 years before the 1857 quake.

Improvements in radiocarbon dating recently allowed Ludwig to process tiny samples she saved from her first dig, refining the dates for the five earthquakes and an average period between the quakes of about 140 years.

Ludwig thought she could find more quakes and more accurate dates if she dug new trenches and took advantage of new dating technologies.

She also thought she could figure out how big the prehistoric earthquakes were.

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