EMERALD HILLS (CBS SF) — The 3.1 magnitude earthquake that shook the peninsula early Friday morning occurred in a rare area, according to the USGS.

The epicenter of the quake, downgraded to a 2.9, was in Emerald Hills, a neighborhood above downtown Redwood City. There have been about 20 earthquakes with a magnitude 3.0 or higher in the area since the 1950s, with the largest measuring in at 4.1.

But a few small quakes in Emerald Hills area were documented in the last month. USGS geophysicist William Ellsworth says it’s unusual to have this many quakes over what’s known as a “locked” fault line, which has been relatively quiet over the decades.

“(The faults) kind of dribble along very slowly over the years,” Ellsworth said.

Tectonic plates normally glide and rub against each other, but periodically they become wedged together. When the fault quits sliding and becomes “locked” in place, it builds energy until it finally ruptures, relieving hundreds or thousands of years of stored-up stress in seconds.

The San Andres Fault, which nearly stretches across the length of California, has several large sections that are locked. USGS seismologist David Oppenheimer said one of the largest sections spans from Loma Prieta to Cape Mendocino. Strain continues to build up in this particular section, making it likely that residents could see a repeat of the catastrophic 1906 earthquake the next time it registers any significant movement.

At this point, Ellsworth said it’s too early to make any conclusions about the section of the San Andreas fault underneath Emerald Hills.

“I think it would be premature to draw any kind of conclusions about any kind of pattern or trend,” Ellsworth said. “We’ve seen small sequences like this over decades. At this point, there’s nothing to suggest that anything’s unusual.”


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