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Scientists Find Link Between South Napa Quake And Summer Months

NAPA (KPIX) - A surprising new theory links the quake that rocked Napa in 2014 with summertime.

The report was published in the Journal of Geophysical Research: Solid Earth. It found that the 6.0 Napa quake may be tied to a dip in groundwater levels that happens during summer months.

The quake struck back in August and was centered just to the south of Napa along the West Napa fault system. Ian Devereux White will never forget it.

"On the inside, all the glass flew everywhere smashed," says White. The sudden shaking jolted him out of bed.

"It's interesting because there's a general awareness it's going to happen again and at some level it makes you really appreciative that it didn't happen today, but it's one of those things you just have to be aware," he says.

The study looked specifically at the West Napa Fault system. Researchers discovered a surprising correlation between earthquakes and a particular season.

"For this particular fault system there is some sort of hazard, increased hazard in the summer months," says geophysicist Meredith Kraner.

Scientists observed a 3 millimeter horizontal expansion of the Earth's crust every summer since 2006. Before it would contract the same amount in the winter. That put increased stress on the West Napa Fault and that helped trigger the 6.0 magnitude quake that rocked the Napa and Vallejo areas.

Researchers said large seasonal variability in the amount of groundwater in Napa Valley and Sonoma may contribute to the "increased likelihood of an earthquake occurring along the West Napa Fault system."

"In addition to the long-term accumulation of friction or stress, we see that there is also a seasonal component that may have pushed the fault slightly more favorably to failure during the summer months," says Kraner.

That leads White to wonder if the old wives' tale about 'earthquake weather' is true.

"But when I gets really hot and windy that's when people say earthquakes come and on really hot days, you really raise an eyebrow," he says.

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