East Bay quakes caused by shift in Greenville fault

Residents were shaken in Contra Costa County Tuesday after a 4.3 magnitude earthquake centered east of Danville near the Los Vaqueros Reservoir. 

The USGS says the first quake registered at 1:11 p.m. Tuesday and was followed by three other small aftershocks.

This quake didn't cause major damage, but it was a scare for some people, with heightened concern following Monday's 3.5 quake in Morgan Hill and the 7.1 Ridgecrest earthquake in Southern California on July 5th.

In downtown Danville, about 15 miles from the epicenter of the quakes, Amaryllis Strohl was starting work at Hartz Cafe and felt the shaking.

"I was just opening and I was wiping the counters...then I felt the entire cafe shake. There's a lot of things moving...it was a little scary," said Strohl.

"That's the scary thing I was home alone. No kit. I was a little bit startled," Brandon Ball, a Danville resident who also works at the cafe.

The quake was on the Greenville fault, which is the same fault that caused a 5.8 earthquake in Livermore in the 1980's according to Bill Ellsworth, a professor of geophysics at Stanford University. 

"The Greenville fault is a fault that moves side to side just like the San Andreas or the Hayward fault. And it is roughly parallel in strike to them. It runs from maybe 10 miles south of Interstate 580 to 10-15 miles of 580," said Ellsworth, "It's a long fault structure so it's capable of  much larger earthquakes. Those in 1980 were about 5.8 and that did quite a bit of damage in the Livermore area, so there's the potential for strong shaking but there isn't anything that would suggest this earthquake is starting a new sequence."

Experts also say it is unlikely it was connected with the Morgan Hill or Ridgecrest quakes.

"We have earthquakes of this size fairly regularly all throughout California so it's unlikely there's a direct relationship between this and the Morgan Hill quake or the quakes in the Ridgecrest area," said Ellsworth.

People throughout the East Bay say the quake had a rolling feel.

Charlene Stark says she was sitting with her husband Ron at their home in Livermore and says it was sudden and severe.

"We were seated and all of a sudden it felt like we were on a roller coaster with someone who slammed in the back of us," said Stark.

"I was sitting at home, it rocked the whole house," said Connor Smith, who was with his mother in Danville.

"The house just started shaking and I knew it was an earthquake immediately," said Emma Bournazos, who lives in Alamo.

Professor Ellsworth says the Greenville fault is capable of producing a quake as large as the Ridgecrest quake but the likelihood is low.

"It's one that's moving at a very slow pace so it would produce a big earthquake thousands of years between them as opposed to the San Andreas or Hayward fault where we're talking hundreds of years between a significant quake," said Ellsworth.

Experts say the light quakes are good reminders to people in the Bay Area of the importance of preparing an earthquake emergency kit.