A look at the Bay Area’s hidden, unmapped fault lines

Bay Area

SAN FRANCISCO (KRON) — Outside of the five major faults in the Bay Area, Research Geophysicist Brad Aagaard with the U.S.  Geological Survey says there are more than two dozen others that are mapped and have the potential to produce at least a magnitude 6.0 earthquake.

There are also unmapped faults, that either have produced quakes smaller than a magnitude 2.0 or simply are not visible from the surface.

The number of those faults is unknown.

Monday night’s 4.5 magnitude earthquake centered near Pleasant Hill happened along an unmapped fault close to the Concord-Green Valley Fault.

About 30 aftershocks have followed.

“Here’s the Concord fault right here, and Monday night’s earthquake was just a little to the west of it,” Aagaard said. “You’ll notice there are earthquakes that have occurred in that region and sort of the trace extension of the Calaveras fault that runs up here toward Vallejo and connects with the West Napa fault.”

He says although it’s unlikely a fault short in length can generate large earthquakes, registering magnitudes 6.5 or higher, it is possible. 

“Some of the faults are sort of relatively well-defined, especially down here in the Calaveras fault,” he said. “But in other areas, it really reflects the complexity of the faults aren’t all vertical, there are some at a dip, some don’t come to the surface, and there’s even minor faults in between.”

Aagaard adds that older faults that are no longer mapped because of a lack of recent seismic activity still have the potential to cause a quake.

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