In-depth: Geologists aim for better understanding of where, when, how often earthquakes will strike


NAPA (KRON) — Since the big earthquake in Napa back in 2014, the USGS has been working on a more detailed map of fault lines in Napa County.

It’s all to help better understand where, when, and how often earthquakes will strike.

QuadCopter4 joined the USGS to see firsthand the work underway.

Deep in the heart of Napa wine country, the United States Geological Survey has been digging trenches 4 feet wide, 16-to-20 feet deep, and 100 feet or more long.

This is an aerial view of one of the trenches taken from QuadCopter4.

Down in the trench, a small team of geologists has been searching for evidence of an earthquake fault line.

Specifically, they have been looking for evidence in the Napa Fault.

“Which is a relatively minor fault, part of the San Andreas Fault system,” Geologist Belle Philibosian said. “It was known prior to the earthquake but was not really studied at all.”

Philibosian is leading this effort.

The earthquake she’s referring to was the South Napa quake, which struck at the southern end of the Napa Fault on Aug. 24, 2014.

That 6.0 quake caused hundreds of millions of dollars in damage across Napa County.

One person died and hundreds were injured. The quake was also a wakeup call for the USGS.

“We’ve learned that we really shouldn’t be ignoring these minor faults,” Philibosian said. “They may have quakes every hundred or every thousand years, but when they do have quakes, we would like to know how often they rupture.”

Prior to the quake, the USGS had only mapped about 22 miles of the West Napa Fault, running from San Pablo Bay to St. Helena.

The trenches they’ve dug sit just north of St. Helena where geologists suspect the fault continues

The USGS chose to dig in this spot because the answer starts with the round river rock which was found on top of a hill.

Eons ago, the Napa River left round cobbles all over the Napa Valley. At some point, an earthquake or series of quakes lift the ground up and created hills in this otherwise flat valley.

The USGS’s trench cut into one of those hills.

Down in the trench, Belle and her team are looking through layers of ancient sediment for evidence of the fault.

Normal sediment layers are horizontal and flat. Whenever the sediment curves or lifts up, that indicates vertical movement of the earth–a sign of past earthquake activity.

When the team is done in the trenches, they will analyze their findings.

The goal is twofold.

First, make a better map of the West Napa fault and then try to predict, based on previous quakes, when the next big earthquake will hit.

“To be able to determine when the last earthquake hit or when the last several earthquakes occurred, and that gives us an idea of how often earthquakes occur,” Philibosian said. “Knowing how long it’s been since the last one allows us to forecast when the next one will occur.”



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