Prepare for a big Southern California quake sooner rather than later, researchers say

UCI seismologists mark research areas at an alluvial fan on the Carrizo Plain with flags. Their goal is to disturb the privately owned farmland as little as possible as they search for more of the San Andreas Fault's history.
UCI seismologists mark research areas at an alluvial fan on the Carrizo Plain with flags. Their goal is to disturb the privately owned farmland as little as possible as they search for more of the San Andreas Fault's history. Molly Peterson/KPCC

For decades, earthquake researchers have sought clues about how the San Andreas Fault has ruptured along a treeless stretch of land near Bakersfield. Seismologists from UC Irvine now say there's evidence of more quakes than they’d previously counted at that spot - findings that suggest a greater potential for the big one in Southern California.

Kern County's Carrizo Plain is starting to shimmer in midday sun, but a woman in a long-sleeved pink shirt doesn't notice. She's penciling field notes into a composition book. Admittedly, a low-tech method. "If I drop it, it still works. If I run over it with the car it still works. It lasts for years and years. Pencil stands the test of time."

Lisa Grant Ludwig is an earthquake scientist at UC Irvine. When she talks about time, she often means it in geologic terms.

The Carrizo Plain is named for the grasslands that used to fill this land between these mountain ranges. Now cattle graze here.

We see a California Condor on the Carrizo Plain national monument. Sand Hill cranes still use the flat as a migratory spot – a dry white lake in the far distance. We're standing pretty much on the San Andreas Fault.

"The last big earthquake here was January 9, 1857," Ludwig says, as we sit in folding lawn chairs. "This area looks pretty much the same now as it did then. We would have a front row seat. We probably would have been tossed out of our chairs and we would have seen like this rip traveling across the landscape as the two sides moved opposite each other."

Ludwig points across the fault toward a small rock pyramid she uses as a marker for an old research spot. Where tectonic plates strike and slip against each other, how far a quake shifts those rocks can help gauge the event's magnitude. "If you can think of a fault as being a fenceline, when the fault moves everything on the other side it's going to shift to the right," Ludwig says. As for the Fort Tejon Quake, again, if it happened today, "that rock pile would move about 6 meters to the right, and there would be a bunch of fresh scarp in between."

The history of earthquakes here is only written in books back to 1857. But the history of earthquakes centuries before that is written in the earth.

UC Irvine scientists have opened up more than two dozen trenches here. They grid and map and take note of the sediment layers so they can see exactly what happened.

The sound of spray paint drifts across the plain.

The can's in the hands of UC Irvine seismologist Sinan Akciz – he's circling and marking trenches marked by old rockpiles. "This one is where trench 11 was," he says, referring to one of two that yielded findings for a new study. "We're labeling our old trenches – we haven't done that for a while, so it's mostly refreshing our memory."

Akciz is the lead author of an article in the journal Geology. It presents evidence that far less time has passed between quakes here than people had believed. "For the last 700 years we had a lot more frequent earthquakes than we previously thought," Akciz says. "We've had six major surface-rupturing earthquakes since about the 1350s, and the most recent one was 1857."

With a cowboy hat low over his eyes, Akciz looks east to the Temblor mountain range. Over centuries, water has spilled onto the plain where we stand, spreading soil on a flat fan of land called the Bidart Fan – named for the spot's owners, longtime ranching and farming operation Bidart Brothers.

Akciz says that soil reveals history. "Each time there's a rainstorm it’s bringing new sediments and its layering them right on top of each other like layers in a cake," Akciz says, gesturing in the air. "And after a couple of these layers deposit, they get disrupted by the earthquake, then new layers cap the event."

The Irvine team's trenches slice into that cake – to see sand, soils and clay. Radiocarbon dating has helped confirm the timing of the events, and narrow the interval window here from several hundred years – up to 400 years between quakes – to the new figure, around 40 to 140 years.

Twenty-two years ago, scientists who forecast San Andreas Fault activity were confident that this part of the fault was least likely to experience a strong rupture in the near future. Ludwig and Akciz now say that was wrong.

"This section was thought to be very simple, very well-behaved," Akciz says. "What we seem to be finding is that earthquakes are not that regular. The time between the earthquakes ranges from 45 years to 145 years."

Akciz and Ludwig add it's likely that the fault, locked as it is now, will release more energy after a longer interval. Chances are the next quake will be big. "What has accumulated since 1857 is the equivalent to what happened during 1857. That we can say. So the longer it waits, the more likely it will be a bigger event."

On this trip Akciz and Ludwig test what could be a faster way to investigate here – ground-penetrating radar. Ron Kaufman pushes a radar unit along the ground. It looks like a yellow and black lawn mower, and it bounces slightly above the crunchy and rocky earth.

"We're basically sending radar signals into the ground," Kaufman says, "and you get a cross-sectional picture of the ground, similar to a marine fish finder, but instead you're using radio waves."

Lisa Grant Ludwig says she feels the urgency of her findings. She wants to shake Southern California into consciousness that a major quake could happen tomorrow.

She says her team will keep digging into the fault's history to learn more about past hazards. "There are dates these events happened," she emphasizes. "There is a right answer. And I may never know exactly what it is. I'll get close to it. But this happened. And I just – I'd like to know."

After the trip, in an email, Sinan Akciz reports ground-penetrating radar won't be able to speed studies. "It looks like the soil is not cooperating with us for this project. It is telling us that we can't cheat. We have to use the old but sure method of trenching."

UC Irvine quake scientists say it’ll simply take time to learn more about this part of the San Andreas Fault.

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