In the quiet predawn hours last Sunday, tens of thousands of Napa County residents awoke to shaking so violent it seemed like an earthquake’s epicenter was right beneath their floorboards.
For many, it was.
The 6.0-magnitude earthquake erupted 7 miles below the earth’s surface in the marshy areas off Milton Road, south of the city of Napa, sending shock waves more powerful locally than any Northern California quake in more than a century.
Seismologists suspect the culprit to be a recently discovered fault of which most Napans were blissfully unaware — the West Napa Fault, a 35-mile-long, subterranean break in the earth that starts in Benicia and runs west of Highway 29 up to St. Helena.
If this proves correct, seismologist have theories to explain why areas of downtown Napa, Browns Valley, and the central part of town were hit harder than others.
U.S. Geological Survey scientists were on the ground in southwest Napa County this week, examining ruptures in the ground measuring several inches wide, in a pattern indicative of the fault line, said Keith Knudsen, deputy director of the USGS’s Earthquake Science Center in Menlo Park.
As recently as 14 years ago, the West Napa Fault slipped and caused a magnitude 5.2 earthquake in Napa Valley. But for seismologists, the West Napa Fault remained a much smaller threat to Napa than larger faults bordering the valley to the east and west.
The Green Valley Fault sits just over the Vaca Mountain range, while the Rodgers Creek Fault sits to the west in Sonoma County. Both are part of the dangerous Hayward Fault system, which last slipped in 1868 and is overdue for a possible repeat, leading U.S. Geological Survey scientists to refer to is as a “tectonic time bomb.”
Last Sunday’s south Napa earthquake was the Bay Area’s biggest earthquake since the deadly Loma Prieta in 1989, a magnitude 6.9 quake the epicenter of which was in the Santa Cruz Mountains. That was too far south to cause shaking in Napa as violent as Sunday’s quake, Knudsen said.
For Napa County, residents have to go back to 1906, when a 7.9-magnitude earthquake hit San Francisco, to find a good comparison point to Sunday’s earthquake, Knudsen said. Even that might not have matched the South Napa Earthquake’s force, although Knudsen said his colleagues will have to analyze the two to see if a legitimate comparison can be drawn.
The best comparison, said Knudsen, was a magnitude-6.3 quake in 1898 that hit near Mare Island and the USGS speculates it may have been caused by the Hayward-Rodgers Creek system, although that hasn’t been definitively proven.
Fifteen years ago, California scientists and experts determined that, by 2030, there was a 32 percent chance the Hayward-Rodgers Creek Fault would rupture and cause a large earthquake of 6.7— to 7.4-magnitude, according USGS.
In the geologic realm, the South Napa earthquake was modest, although residents who suffered the brunt of its seismic force may find that hard to believe.
“This area is bordered by bigger, more active faults,” Knudsen said. “This should be a bit of a shot across the bow that they should be prepared.”
The South Napa earthquake undoubtedly brought back memories of the 2000 quake to some residents. Both quakes hit early on a Sunday morning. The jolt struck at 1:36 a.m. on Sept. 3, 2000, a 5.2-magnitude earthquake with an epicenter near Yountville. Last Sunday’s magnitude 6.0 hit at 3:20 a.m.
The 2000 quake was attributed to the West Napa Fault, but initially USGS scientists believed it was an unmapped, undiscovered fault system that was responsible. It took the protests of the geologist who discovered the West Napa Fault in 1976, Gene Boudreau of Sebastopol, to convince them otherwise.
Boudreau specializes in groundwater drilling, and in the process of drilling a well off of Dry Creek Road northwest of Napa one day almost 40 years ago when he found something that didn’t quite fit with contemporary geologists’ mapping of that area, he said in an interview Friday afternoon.
In drilling down, Boudreau uncovered a contrast of newer, volcanic rocks with a basement of older rocks. He suspected a fault explained the contrast, and investigated further, looking for cracking and ruptures in the ground that would form a pattern and provide evidence of a fault line.
He found what he was looking for in cracking on Oakville Grade Road and on Old Sonoma Road, and notified the USGS. Seven years later, its scientists confirmed his finding and published a report.
“I figured there had to be a fault,” Boudreau said. “It must be active. Just because it’s active doesn’t mean you have an earthquake every day.”
Boudreau said he saw similar patterns of cracking in 2000, and again after Sunday’s earthquake. That quake caused an estimated $50 million in damage, and affected thousands of buildings, although only a handful of people were injured.
“Everything was in the same place,” Boudreau said of the similar patterns of cracking. “The cracks were where I said they were. Everything I said has been proven true.”
Two earthquakes in 15 years on the same fault line should be enough of a warning sign to residents and elected officials that more precautions, planning and land-use planning changes should be undertaken, he said. In geology, earthquakes on an active fault are a question of when, not if.
“They think if it happened, it won’t happen again,” Boudreau said. “It’s going to happen again. We don’t know how many times it’s happened in the past. This fault’s not going to go away. There’s going to be more moving.”
Knudsen said USGS scientists are developing theories as to why downtown, Browns Valley and central Napa were hit so much harder than other areas. He said there are three factors at work, and potentially a fourth.
The first is the proximity to the quake’s epicenter, and Napa was a scant 6 miles away. Older buildings, including Victorian homes as well as masonry and other stone structures, hold up the worst in the violent shaking. Downtown and central Napa has an abundance of such structures.
The third is the geologic conditions beneath the earth’s surface. While the Napa Valley has a fair amount of bedrock in its hillsides and foothills, according to a 2008 USGS report, the areas beneath downtown and central Napa are different because they hug the Napa River.
Knudsen said the river means these areas have alluvial sediment that doesn’t buffer the seismic shockwaves from the earthquake as well as bedrock would, causing more building cracking and damage.
“Napa’s on soft soils,” Knudsen said. “It’s relatively close to the epicenter.”
The fourth potential factor is that earthquakes, depending on how the plate tectonics act in slipping, do not send out seismic shockwaves in concentric circles like some people would believe.
In the South Napa earthquake, Knudsen said scientists suspect the greatest energy unleashed by the quake happened on a south-north direction from the epicenter. That would place Browns Valley, central Napa and downtown Napa squarely in line to receive a greater force.
“It can have a directionality to it,” Knudsen said. “That fault can rupture all the way to the surface. They are finding small amounts of surface rupture.”
Steve Pryor, a structural engineer in California, said owners of older-model homes should ensure they’re sufficiently retrofitted to withstand an earthquake larger than Sunday’s. One of the basic ways to do so is to shore up a home’s cripple wall to keep it from sliding off its foundation.
“Every earthquake unfortunately teaches us some of the same lessons,” Pryor said. “This was a fairly moderate earthquake. This was not the Big One for Napa.”