Researchers detail Napa earthquake's patterns, and lessons
Lisa James/Register
The South Napa earthquake produced rare fissures. Such surface eruptions hadn't been seen in a Northern California earthquake since 1906, researchers said. This one occurred at 3:20 a.m. on Aug. 24 on Meadowood Court in Browns Valley,
If your home, driveway or pool were damaged in the South Napa earthquake, should you make repairs immediately – or wait until the local fault lines are better understood?
That was the question on the minds of audience members Monday night during a forum outlining new data emerging from the Aug. 24 quake. Speaking to more than 60 people at City Hall, members of the U.S. and California geological surveys explained why the temblor damaged homes and buildings in the manner and locations it did – and what lessons the experience holds for homeowners, engineers and city officials.
Jack Boatwright, a USGS representative based in Menlo Park, reported the largest share of Napa buildings given yellow tags restricting access, or red tags banning occupancy, were built before 1950 and not designed to statewide seismic codes first passed 40 years ago. More heavily damaged structures were concentrated not only downtown but in the city's southern half and Browns Valley to the west, largely due to weaker soil in the sedimentary basin underlying Napa, he said.
"So much of what the public has seen of the earthquake has been from downtown, but the effects are seen citywide," Rick Tooker, the city's community development director, told the audience.
Brick chimneys were especially vulnerable, accounting for about half the total tags, said Tim McCrink, engineering geologist for the California Geological Survey. Rubble from chimney collapses also seriously injured a Napa teenager and blocked a sidewalk along Jefferson Street west of downtown.
"I moved to Napa in 2000 – into a house that already had a cracked chimney from the 1999 Yountville earthquake – and I took the chimney down," he said. "I think it's time to stop building brick chimneys, or to pretend we can build them (here) in a safe way."
Different foundation designs also led to greatly different outcomes for homes even in the same neighborhoods. While homes with shallow foundations often detached from the ground, and cracks weakened other houses by passing between piers, so-called moment frames with horizontal beams firmly attached to columns generally withstood the seismic shock best, according to Julien Cohen-Waeber, a researcher of environmental engineering at UC Berkeley.
Though the Napa quake's magnitude of 6.0 paled in comparison with the Bay Area's major temblors of 1906 and 1989, it left dramatic traces rarely seen after seismic events of its level, Cohen-Waeber said.
The ground fissures produced by the West Napa Fault are the first in Northern California since the 1906 quake that leveled San Francisco, and the first in the region to slice through developed areas, he said. Furthermore, the post-quake motion known as afterslip has moved the fault another 4 inches since Aug. 24 on a stretch of fault line zigzagging around and through dozens of homes amid the cul-de-sacs of Browns Valley.
"We hadn't had to deal with afterslip going through a neighborhood before – anywhere in the world, actually," said Ken Hudnut, the USGS geophysicist who wrote the report detailing the fault's post-quake motion that is expected to continue but slow down for at least three more years.
The USGS report declares afterslip a moderate threat to only about 20 Browns Valley houses, but new findings on the city's network of fissures may affect where future homes are built elsewhere.
Perhaps the biggest uncertainty for several homeowners in the audience was the effect of the new seismic hazard maps that will emerge from the data drawn from the August earthquake – an effort that could show fault lines passing through buildings and curbing new construction on their paths.
The state Alquist-Priolo Act from 1972 requires the mapping of active faults in quake-prone regions of California. While the law does not prevent repairs to single-family homes in such hazard zones, subdivisions of land along faults are subject to building curbs.
Afterward, some audience members suggested they would delay repairs until revised maps show more precisely where Napa's more quake-vulnerable zones are, and where expanded hazard areas might restrict new construction.
Fred and Kathy Wood, for instance, were ready to postpone fixing cracks the temblor opened in the swimming pool behind their Browns Valley house.
Despite the damage, however, Fred Wood came away reassured that the quake's aftereffects wouldn't further threaten his home, which lies east of the West Napa Fault's main trace.
"I feel more secure now because we know we're not right on the fault, so this slipping won't affect us," he said.
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