Science on Napa earthquake could improve damage forecasts
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.