Using computer models, a catastrophic risk management firm estimates the south Napa earthquake caused an estimated $1 billion in damages — more than twice the amount of earlier local estimates.
Ten percent of homes in Napa’s Browns Valley and 20 percent of residences in the downtown area were damaged in the Aug. 24 earthquake, according to a report by Karen Clark & Co., a firm based in Boston.
According to Karen Clark & Co., 40 percent of the $1 billion in losses was residential, and the rest was from both commercial and industrial buildings.
Karen Clark & Co. came up with the $1 billion estimate based on such parameters as the quake’s epicenter, magnitude and depth. Of that $1 billion, only $100 million is covered by insurance, the company said in its report.
Engineers from the company traveled to Napa within a week of the quake with the goal to measure the amount of loss and damage, said Clark.
The company’s estimated damage total for the quake is substantially higher than early estimates by local sources. Overall, as reported by the city, more than 1,500 residential units were damaged in the quake, with close to 10 percent suffering severe damage or complete destruction. In total, 130 buildings were red tagged and 1,182 yellow tagged by city building inspectors.
Local government previously estimated losses from the quake at $362 million, with an additional $80 million to $100 million in losses to the wine industry, bringing the countywide total to more than $442 million.
Because of the magnitude of the quake’s damage, Napa was declared a state and federal disaster site and made eligible for loans and grants from the Federal Emergency Management Agency and the U.S. Small Business Administration.
Individuals and business owners have until Dec. 29 to register for disaster assistance with FEMA. The agency has already served 2,500 homeowners, renters and business owners at the Local Assistance Center, 301 First St., Napa.
Napa’s valley floor location was partly to blame for some of the destruction, said the report. Because much of the city lies within a valley lined with soft river deposits from the Napa River, those deposits amplified the ground motions of the quake, “resulting in disproportionately higher damage” compared with surrounding towns.
The inspectors said they found a range of damage from minor to severe.
In Browns Valley, approximately 10 percent of single family homes were damaged, with the greatest damage observed closer to the fault “trace,” or where there was surface evidence of the fault rupture, such as cracks in the pavement or concrete, Clark said.
Homes in Browns Valley are primarily wood-framed structures of one or two stories. Most built after 1960, under higher construction standards, showed less vulnerability to earthquake shaking.
Damage to foundations caused settlement and tension cracks in walls, said the report. “Significant structural damage was observed at homes along the fault, leading to larger cracks in the streets and garages and other structures shifting off foundations. Chimney damage was widespread, including partial and complete collapse.
“Most of the chimneys not affected had been repaired due to damage from the 2000 quake. Damage to stucco walls was also common, mostly occurring at window and door openings.”
The engineers found more of the same in downtown Napa, they said. But unlike Browns Valley, more than half the downtown area structures were built before 1960, and many before the 1940s. Approximately 20 percent of homes in the city’s core were damaged, mostly “due to the older building construction” and softer soil conditions.
The most severely damaged downtown homes included those with “cripple” walls. Cripple walls are short wood-framed walls that support a dwelling between the foundation and first floor, used to create a crawl space under a home. If a cripple wall is not adequately braced, it can shift during a quake and result in major structural damage. Cripple wall bracing was not required by California building code until the 1970s, noted the report.
The company estimated 60 percent of Napa Valley wineries received some level of damage.
The engineers also visited neighborhoods in American Canyon where they found that just a few homes per hundred were damaged.
Damage to homeowner and business contents was prevalent, “even to homes with no evidence of building damage,” said the report. Even with low levels of ground motion, fragile contents on bookshelves, cabinets and drawers are easily damaged, it said. Not surprisingly, the most significant contents damage was observed in downtown Napa and Browns Valley.
Commercial structures were also studied. The highest frequency of damage was observed in downtown Napa and included broken windows, cracks in siding, and damage to roofs and siding. There was significant damage in restaurants, grocery stores and wine shops, the report noted. Even with low levels of ground motion, “signage can be vulnerable, as plastic panels can shatter or pop out of frames.”
The engineers paid specific attention to the Franklin Street post office. Built in 1933 out of unreinforced masonry, the building was significantly damaged, receiving a red tag from the city. The brittle mortar of unreinforced masonry is “particularly vulnerable” in earthquakes, said the report. Glass window panes such as those found at the post office were also vulnerable to shaking as well as falling bricks. Falling debris from the post office was a “significant contributor” to additional damage at the building.
The Goodman Library was retrofitted in 2007, but “the effectiveness of the retrofit varied considerably,” said the report. The decorative roof details seen at the very top of the front of the historic building “did not receive adequate attention” in the retrofit process “and created a significant falling hazard.” That building remains under repair.
One of the most badly damaged buildings in downtown, the Alexandria Square building, was built in 1910 and retrofitted in 2004. An entire section of the corner cupola collapsed, “likely a result of deteriorated mortar between bricks which caused points of weakness in the structure, resulting in partial building collapse,” said the report. The repair is estimated to cost between 30 and 40 percent of the building’s value.
The two inspectors found that new construction was not immune to the shaking.
Clark said the inspectors were surprised in part to see there was “a lot of the damage to even some of the newer buildings,” such as the Andaz.
Almost all exposed wall surfaces of the hotel, built in 2009, were cracked or collapsed, it said. “Even in new construction that meets current building codes, siding and decorative features can be vulnerable to earthquakes.”
More than half of wineries in Napa Valley were impacted by the quake, said the report. Damage included wine storage barrels, winery buildings, warehouses and infrastructure. Most of that damage occurred north of downtown Napa.
Images of a dramatically leaning building at Trefethen Family Vineyards flooded the Internet after the quake. The three-story wood-framed building was built in 1886, but no evidence of seismic retrofitting was observed, said the report. It appears to have shifted off its foundation.