M6.0 Earthquake is California Bay Area’s Strongest Since Loma Prieta

A magnitude 6.0 earthquake struck near Napa, Calif. early Sunday morning is the strongest earthquake to hit the San Francisco Bay area since the 1989 Loma Prieta earthquake, which registered a magnitude 6.9, according to catastrophe modeling firm AIR Worldwide.

The shallow American Canyon quake at a depth of 6.7 miles struck at 3:20 a.m. PST and lasted 30 seconds, according to AIR.

The quake was felt widely throughout the region. Gov. Jerry Brown declared a state of emergency for southern Napa County, and ordered state agencies to respond with equipment and personnel.

The earthquake sent at least 87 people to Queen of the Valley Medical Center in Napa, where officials set up a triage tent to handle the influx, the Associated Press reported.

There were reports of structural damage to unreinforced masonry buildings in Napa’s historic downtown, according to AIR.

Pacific Gas and Electric reported power outages to roughly 10,000 households in Napa and surrounding towns, and the California Highway Patrol was checking bridges but there was no immediately reported no structural damage.

The California Earthquake Authority estimated roughly 15,000 of its policyholders may have experienced moderate to strong shaking. The percentage of homeowners and renters who have earthquake insurance in the affected area is low. In Napa less its’ than 6 percent and in Sonoma its less than the 10 percent statewide average, according to CEA.

According to AIR, the quake had a strike-slip mechanism with a nearly vertical dipping angle.

“The epicentral location is very close to the West Napa fault, which lies between two major traces of the San Andreas fault system — the Hayward-Rodgers Creek fault zone to the west and the Calaveras-Concord Green fault zone to the east,” AIR stated.