Image courtesy USGS

The magnitude-6.0 earthquake was centered between the communities of American Canyon and Napa in Northern California.

Image courtesy UC Berkeley Seismological Lab

The ShakeAlert early warning system at UC Berkeley detected the quake 10 seconds before it occurred.

It was the strongest trembler to hit Northern California since the Loma Prieta earthquake struck the San Francisco Bay Area in 1989.

“The damage is not as bad as it could have been,” says Mark Ghilarducci, director of the Calif. Governor’s Office of Emergency Services. While there was some road buckling and pavement cracking in the area, Caltrans certified that all bridges are safe, he says. Around 100 homes were red-tagged, meaning they are unsafe to enter, due to damaged foundations or chimney collapses.

One such chimney collapse injured a 13-year-old boy, who was flown to a trauma center in the Bay Area for further treatment. In all, there were 178 emergency room visits to Napa’s Queen of the Valley Medical Center in the hours after the earthquake, but only 13 of those were admitted, says Walt Mickens, the hospital’s president and CEO, during a media briefing. The totals were revised to 208 seeking treatment and 18 admitted to the hospital by Tuesday morning.

In Napa, the area’s largest city with just over 78,000 residents, crews are completing repairs to 90 broken water mains that had cut off water to around 600 people. Many residents lost power Sunday, but by Monday virtually all power was restored. Around 100 suspected or confirmed gas leaks were resolved by PG&E, according to the city.

Napa County Airport's control tower is inoperable after the quake shattered most of its windows. Flights continue in and out of the airport, however, with pilots communicating their positions to each other directly via radio.  

Napa fire operations chief John Callanan says crews responded to four fires, the largest at the Napa Valley Mobile Home Park. A nearby water main break meant fire crews did not immediately have enough water to put out the fires, and six mobile homes were damaged or destroyed. No one was injured in the mobile home fires, and no commercial structures suffered fire damage due to the quake. 

Within the Napa city limits, 33 commercial and residential structures have been red-tagged during the initial assessment, says Rick Tooker, Napa community development director. Around 20 to 30 building inspectors and structural engineers will complete inspections of all buildings within the city limits on Monday. Downtown, bricks tumbled onto city streets from a partial collapse of the exterior of a three-story retail and office building at the intersection of Brown and 2nd streets. Nearby, a historic courthouse built in the mid-1800s was partially condemned due to façade damage on its eastern side. Napa's First Unified Methodist Church suffered significant damage inside and to its façade.

While most of the damage occurred to unreinforced masonry buildings, there was also damage to recently retrofitted and newer structures, including the Andaz Napa Hotel, built in 2006.

“Some of the buildings that were retrofitted didn’t fare as well as you might expect, even with recent work,” Tooker says, adding that once the initial safety assessments have been completed, engineers will determine the reasons behind the structural problems in the retrofitted buildings and what to do about them.