Ruptures on the surface, including sections of pavement that buckled, to thedelight of area skateboarders, are now helping scientists map these fault lines, shedding insight into the area’s earthquake risk exposure.
According to the U.S. Geological Survey (USGS), ruptures occurred along known faults, old faults, and areas where no faults had been mapped before. It’s not yet clear which fault caused the earthquake, though geologists suspect the Browns Valley branch of the West Napa Fault.
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USGS geophysicist David Schwartz’s team of geologists are taking tape measures and GPS units to the ruptures and cracks in Napa’s roads to map the previously unknown fault. They’ve determined it runs to the north and northwest, and that the quake caused a lateral slippage of 2.5 inches. The east side of the fault moved to the south and the west side went north by that amount, Schwartz says. (Image below is not a member of Scwartz’s USGS team.)
Agency scientists have been mapping what they’ve observed, with the blue pins indicating surface ruptures, and the orange lines show the active West Napa Fault. The yellow lines show 130,000-year-old faults, and the blue show 1.6 million-year-old faults.
Schwartz says the active faults of the Napa valley are “not very well known, they haven’t been very well mapped,” partly because they are the “weak sibling” of California’s larger and better known faults.
Geologists identify active faults by their unusual surface morphology, like the distinctive gullies of the San Andreas Fault, which become obvious when viewed from the air. When geologists suspect a fault exists, they look for evidence of movement. But the Napa Valley’s faults are subtle and move more slowly, Schwartz said, so many geologists never knew they were there.
What’s more, floods in the past 2 million years deposited sediment in the Napa Valley and obscured the surface evidence, says Chris Wills, supervising engineering geologist at the California Geological Survey, which maps the state’s geology, seismology and mineral resources. Suburban development in Napa also hid surface evidence.
The flood sediments, as well as other unique geological features of Napa Valley, make the area perfect for growing grapes. The local wine industry was hard hit by the quake, which scared tourists away during a peak weekend and smashedcountless bottles.
Not only did Napa residents get hit with a quake from an unfamiliar fault line, but they also experienced more shaking because the area’s land tends to amplify land motion.
“The local geology really made the damage worse,” Schwartz says. Napa Valley has a high water table with saturated sand and silt deposits, he said, and this amplified the tremors and heightened the damage.
Wills says a survey of the area’s bedrock had suggested the presence of an active fault, and his agency had intended to evaluate it as a possible surface rupture hazard zone. “It was on the list of things we knew we needed to look at,” he says.
However, funding cuts meant the California Geological Survey simply didn’t have the manpower to investigate every possibility. Those same shortages caused the late discovery of the Hollywood Fault earlier this year.
As a result, some buildings were built directly on top of the fault that caused the Napa quake. California law prohibits construction on surface rupture hazard zones.
Schwartz says a subdivision was constructed on the fault and sustained significant damage, which likely wouldn’t have happened had geologists known that a fault was there and that it was active. The vast majority of homes and businesses in Napa did not have earthquake insurance and this research could also help spur new efforts to bolster building codes and insurance requirements.