The two Sunday earthquakes that jolted parts of the Midpeninsula and were centered in Los Altos occurred at a depth considered "relatively shallow" in the Bay Area -- 3.5 km (2.1 miles), a seismologist from the U.S. Geological Survey (USGS) reported Monday morning.
The first quake, a 3.1 magnitude at 9:28 a.m., emanated under Briarwood Court, which is northeast of the Rancho Shopping Center. It was accompanied by two aftershocks of magnitude 2.0 less than 0.2 miles away, near Covington Road and Riverside Drive, seismologist Steve Walter stated in an e-mail.
A map of seismic activity also showed a smaller shock hit Sunday just east of Interstate 280, where the fault plane surfaces, near Dawnridge Drive.
Sunday's second quake -- a 2.4 magnitude -- occurred at 6:38 p.m. with an epicenter along Altos Oaks Drive, less than 0.3 miles from the morning quake.
Due to their depth, the quakes were determined to have originated in the Monte Vista fault zone, Walter said.
"The Monte Vista fault zone is part of the larger Foothills Thrust fault system that bounds the southwest margin of urban Santa Clara Valley," Walter wrote in an e-mail, quoting a paper by Vicky Langenheim and others.
A seismic map shows the plane of the Monte Vista fault zone surfaces along 280 and branches into forks on both the east and west sides of the highway. It reportedly runs through the Foothill College campus.
Although residents were startled by the earthquake, there was little if any damage.
Kathy Hobson, a resident on Briarwood, said described the morning temblor as "a loud bang."
"My dog came and looked at me and said what's going on? The lights were swinging, but nothing fell off anything," she said.
Another resident on the street, who did not want to be named, said it "sounded like a freight train" when the quake hit.
"It shocked me out of my chair," she said.
Click here for a map containing local fault lines.