'Trenching' can make or break quake zone development

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Earlier this year, the state released a preliminary map of a 10-mile long earthquake fault running through parts of Hollywood.

Now, developers looking to build in a 1000-foot area around the fault must first prove their structures won't lie directly on top of that potentially dangerous fracture.

It turns out, the best way to do that is to dig a really deep hole.

Related: State geologists release map of Hollywood Fault

The technique is called “trenching,” said  Kate Scharer, a researcher with the US Geological Survey.

Scharer recently dug a trench on a portion of the San Andreas Fault near Elizabeth Lake, about an hour and a half north of Hollywood.

"What we are trying to understand is how often large earthquakes happen on the San Andreas Fault," Scharer said.

That trench is about 3 feet wide and 12 feet deep. It has scaffolding inside to help prop up the dirt walls and allow researchers to climb up and down the ditch.

"When we excavate these trenches we see layers in the ground that are basically horizontal," she said.

Those horizontal layers are different types of sediment deposited at different periods in history.

The layers closest to the top are only a few decades old; those at the bottom date back hundreds of years.

Scharer said you can tell when a major earthquake happened because there will be a tear in the layers. One will suddenly drop by several inches, creating a visible deformity in the wall.

"And we can follow that deformation and try and understand when that happened."

Her team does that by taking samples from the deformation and using radiocarbon dating to figure out when the rupture took place.

"What we do is essentially exactly the same," said Michael Reader, CEO of Group Delta, the company hired by the development company, Millennium Partners, to dig a trench on land in Hollywood to determine if a fault runs underneath it. Millennium wants to build a pair of skyscrapers at the site. 

Reader said the Hollywood Fault is not as easily delineated as the San Andreas because it's not as well defined. 

His team has dug trenches on two different sites located in a zone state geologists have said is likely to contain traces of the fault. But so far, the Group Delta researchers haven’t found any evidence of quakes in the last 11,000 years. That’s the time frame required for a fault to be considered active.

“So what we are starting to find is where it is not," Reader said. "The question is where is it?”

That’s something a lot of other developers want to know as well.

State law defines an earthquake zone as the 500 feet on either side of a presumed fault line. So any developers looking to build in the area around the Hollywood fault must first do trenching or a similar sub-surface investigation of their own. That could cost tens of thousands of dollars and significantly delay construction.

Tim McCrink is with the California Geological Survey, the state agency responsible for mapping earthquake faults.

He says all of this points to how hard it is to pinpoint the exact location of a past rupture.

“The geological record is kind of an imperfect book," he said.

For example, he points out that in Hollywood, there are areas with very little fresh sediment to dig through. That means there might not be enough geological evidence to determine what seismic events may or may not have happened there.

Past development has also complicated the soil record in the neighborhood.

Even though trenching is the gold standard when it comes to revealing the history of a fault, the technique is still limited, McCrink said.

“You can put a lot of time and effort in to that kind of study and still end up with inconclusive results."

McCrink says the California Geological Survey will aggregate trenching results from developers like Millennium Partners. They’ll use that information to update their map of the Hollywood Fault zone and release a final version later this summer.

 

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