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Tim Whitlock said he could hear the earthquake coming before it pummeled his west Napa home, rocking the structure from its foundation and illuminating the bedroom with a green glow the retired pilot later attributed to static electricity.

When Whitlock and his wife, Ann, went downstairs the morning of Aug. 24 last year, the couple was shocked to discover a scar 6 inches wide running through the living area.

“It looked like a torpedo had been shot out of a submarine through our house,” said Tim Whitlock, a retired Air Force and American Airlines pilot.

The Whitlocks didn’t know it then, but their Twin Oaks Drive home in the Browns Valley neighborhood is situated on a newly discovered strand of the West Napa fault, the geologic rift that runs roughly 27 miles from St. Helena to American Canyon and was blamed for the magnitude-6.0 temblor that rocked Wine Country one year ago. The quake factored in the death of one person, injured about 200 others and caused about $360 million in property damage throughout the region.

Scientists studying seismic activity in the area, from the ground and the skies, believe the strand, or fault line, could radically change our understanding of earthquakes and possibly lead to forecasts for predicting future earthquake damage. The work could have statewide ramifications, not just for homeowners, but for public agencies looking to safeguard transportation and utility infrastructure and companies overseeing vast subsurface pipeline networks.

The scientists’ work is focused on a concept known as “afterslip,” observed for the first time in U.S. history in a residential neighborhood following the South Napa earthquake, according to Ken Hudnut, a top federal scientist documenting the phenomenon. The term describes the slow and often prolonged movement on a fault following an earthquake.

Based on observations to date, Hudnut and his team with the U.S. Geological Survey issued the first-ever forecast for gradual shifting along the newly discovered fault line, which runs roughly 12 miles from Leaning Oak Drive north to Partrick Road in Napa.

Hudnut predicts the fault line will move 2 to 6 inches over the next three years, creating a moderate hazard to about 20 homes on its direct path.

While that may not sound like much, such slippage could prove problematic for homeowners who make repairs calculated to exact measurements.

Another area of the fault line is forecast to continue slipping at a slower rate of no more than 2 inches in three years. About 200 residences are located within that area.

The total value of the homes that may be affected by afterslip across both zones is roughly $110 million, based on an average home value of $550,000, according to a USGS report. Hudnut is a geophysicist for the agency.

The research could raise potentially troubling questions for homeowners in the area as they continue to undergo repairs of damage from the quake. The findings have become part of a regular city advisory to Napa residents seeking construction permits.

Tim Whitlock said he spent about $400,000 from his retirement savings to repair and reinforce his house. The 2,900-square-foot home was lifted about 6 feet in order to install a new foundation.

The work could be threatened by continuing slippage along the fault line, or by another earthquake. But Whitlock calculated that the money he spent on repairs is less than what it would cost for him to purchase a new home in Napa.

“I don’t think lightning hits twice in the same place,” he said recently while standing inside his remodeled home. “I don’t know. Hopefully it (the earthquake) relieved enough pressure.”

Across the street, construction work continues on another house that also is situated along the fault line. Whitlock said the elderly couple who lived there moved after selling the home well below market value. They apparently didn’t want to go through the expense and trouble of doing renovations.

Napa building officials are providing information about the afterslip forecast when residents apply for construction permits. Several homeowners also attended a public forum in January hosted by the city to discuss the projections.

Following the 1971 San Fernando earthquake, a state law was enacted in California prohibiting construction of human dwellings in areas where surface ruptures have been observed and documented.

Surface rupture, like that observed in the Browns Valley area of Napa following the August quake, occurs when movement on a fault breaks through into the open.

The phenomenon is not always a given for quakes. The powerful Loma Prieta earthquake in 1989, for instance, did not result in surface rupture, despite wreaking havoc across the Bay Area.

Then again, earth scientists did not have the sophisticated tools they do now to locate, map and track more obscure earthquake phenomena. The South Napa quake has brought out all the latest toys.

Last week, Hudnut returned to Napa to lead a team of researchers on another survey of the fault line, including reconnaissance from the air. The team flew in and out of Napa County Airport daily to map the area, using sophisticated laser and computer equipment mounted inside a twin-engine Italian-made plane.

At the controls was Barry Hansen, chief pilot for Aspen Helicopters, Inc. and a retired Marine Corps helicopter pilot. The crew, which alternated taking turns flying in the small plane, included Adam LeWinter with the U.S. Army Corps of Engineers and Craig Glennie and Darren Hauser, both with the University of Houston’s National Center for Airborne Laser Mapping.

The plane has a bomb bay that opened to reveal the laser. From a height of about 1,500-feet, the researchers flew over and over across the fault line. They called it “mowing the lawn,” a back-and-forth method used to capture mapping data.

The laser generates 400,000 pulses a second to create a high-resolution topographical map, which Hudnut expects will show additional “creep movement” along the fault line consistent with his projections.

Hudnut, who attended the January community meeting in Napa, said his role is to provide the best available scientific advice.

“We worked really hard to provide accurate forecasts, and then from that point, it’s like take it or leave it,” he said.

The findings have potential ramifications well beyond Wine Country.

“Being able to predict the total amount of afterslip and how fast that it will occur is of a lot of interest to agencies running infrastructure, like BART tunnels or pipes, across the fault,” said Tim Dawson, engineering geologist for the California Geological Survey.

Hudnut singled out the Hayward fault as being of particular interest for such forecasts. He said a magnitude-7.0 earthquake on that fault running through Oakland and Berkeley could trigger afterslip of more than 6 inches within three years of the temblor.

“Now that we’ve gone through all of this in the West Napa system, we feel like we’ll be better prepared to make an accurate, timely forecast (of afterslip) for future earthquakes,” he said.

At his home, Tim Whitlock did not seem particularly worried about potential risks to his home.

He said last August’s earthquake taught him that possessions are not what counts in life. Of the many items he and his wife lost in the disaster, the only thing the couple had insured was a china set they never used.

“It makes you realize things aren’t important,” Tim Whitlock said. “People are.”

You can reach Staff Writer Derek Moore at 521-5336 or derek.moore@pressdemocrat.com. On Twitter @deadlinederek.

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