The earthquake that struck Napa County early Sunday could have been much larger, scientists said, given the track record of the fault that most likely caused the shaking.
Tom Brocher, director of the Earthquake Science Center at the U.S. Geological Survey in Menlo Park, said officials have not yet determined exactly which fault caused Sunday’s event. The magnitude-6.0 quake was centered about one mile west of the Napa County Airport, in a marshy area near Milton Road.
Brocher said the “leading contender” is the West Napa Fault Zone. It is called a fault zone because it includes a number of geologic features that run generally north-south along the west side of the Napa Valley.
“We’re still trying to get the smoking gun,” Brocher said. “As we speak, we actually have crews in the field who are trying to find evidence for surface rupture produced by the quake. That would be a good indication of the earthquake fault.”
Larger quakes have occurred in the region of the West Napa Fault Zone in the past. For instance, a magnitude-6.3 earthquake occurred in 1898 near Mare Island, just south of Sunday’s temblor. The West Napa Fault has triggered about seven quakes of at least magnitude-5.0 since 1969, said John Rundle, a professor of physics and geology at UC Davis.
“It’s a pretty active fault system,” Rundle said.
Tom Rockwell, a seismologist at UC San Diego, said Sunday’s quake should be considered a “wake-up call” for the Napa Valley wine industry, which lost significant amounts of inventory in Sunday’s quake.
“Basically, this could have been a much larger earthquake,” said Rockwell. “What I mean by a wake-up call is I think it’s important for the industry up there to realize they do have an active fault that goes up the valley. It could produce earthquakes that are even larger than this.”
He said one problem in assessing earthquakes in the region is that wineries have been reluctant to let geologists dig trenches on their land to test the soils and assess faults in the area. That’s because they don’t want their vineyards disrupted by digging. “And that’s understandable,” Rockwell said.
The West Napa Fault is sandwiched between two larger faults: the Hayward Fault and the Concord-Green Valley Fault. All these, in turn, are part of the larger San Andreas fault system, and they sit along the seam where two massive continental plates in the Earth’s crust rub together.
Sunday’s fault was considered a “strike-slip” earthquake, meaning the two sides of the fault slid past each other, the west side moving north relative to the east side.
“In many ways, it was a fairly typical Bay Area quake,” said Brocher. “It’s not a huge surprise.”
Yet it seemed to cause lots of destruction for a quake that barely achieved a “strong” rating, which includes quakes between 6.0 and 6.9 on the Richter scale. There are three reasons for that: The epicenter was fairly shallow (6.6 miles deep), close to populated areas and it struck in a region with lots of sedimentary soils, which can increase shaking intensity.
“It did a surprising amount of damage considering it was only a magnitude-6,” Rockwell said.
With earthquakes of this sort, there is at least a 50 percent chance of aftershocks exceeding magnitude 5.0. There is also a 5 percent chance that Sunday’s quake was actually a “foreshock,” meaning an even bigger quake is still coming within the next few days.
According to the USGS, there have been several dozen small aftershocks already, mostly in the 2.0 to 3.0 range.
Sunday’s event was the largest to strike the Bay Area since the devastating Loma Prieta earthquake of 1989. Such earthquakes are often known to relieve stress in fault systems, potentially avoiding a larger quake later. But Rundle said that probably isn’t the case this time.
“I’m surprised it’s been as quiet as it’s been,” said Rundle. “What this probably did is load up even more the Hayward Fault, which brings that guy closer to failure. We’re sort of entering the window when that might occur.”
The Hayward Fault, which runs approximately from San Jose to San Rafael, is considered the most likely source of a major earthquake in the Bay Area. The USGS estimates there is a 31 percent chance of a magnitude-6.7 or greater earthquake on the Hayward Fault over the next 20 years.