The U.S. Geological Survey says an earthquake with a preliminary magnitude of 4.0 hit at 2:13 p.m. Sunday.  That quake was later downgraded to a 3.6 by the USGS. It was centered a mile south of Concord, along the Concord fault.

A series of smaller quakes shook the area in the moments before and after the larger one.  A magnitude 2.5 earthquake hit at 2:01 p.m., followed by the 3.6 at 2:13, and a 2.7 magnitude quake at 2:14 p.m.  A magnitude 2.4 shook the area again at 2:28 p.m.  A magnitude 1.6 hit at 2:56 p.m.

A swarm of quakes, one measuring 4.0, struck the East Bay.

The earthquakes struck at a depth of 8 to 9 miles.  Deeper quakes are less noticeable while a shallow quake–in the 0 to 40 mile depth range–can feel much stronger than their actual reported magnitude. Quakes just below the earth’s surface, in the 0 to 10 miles range, can cause even more damage at lower magnitudes.

“A four’s just sort of a nice little warning that you’re dealing with something that has the potential to be bigger,” USGS Seismologist Dr. David Schwartz said.

Sunday’s quakes may have ruptured a water line four miles away in Walnut Creek that was reported leaking about an hour after the shaking stopped.

Thousands of Bay Area residents reported feeling the shaking on the USGS Did You Feel It site, and on social media sites like Twitter.

Irish Girl tweeted, “Nothing like an Earthquake to shake up a lazy Sunday!”

Yo~ongie tweeted, “@BamBam1A did you feel the earthquake? Welcome to the state of Earthquakes.”

In Southern California, a magnitude of 3.9 hit the Los Angeles area at 4:07 a.m. Sunday. That quake was centered a mile northwest of the View Park-Windsor Hills neighborhood, just north of the cities of Inglewood and Culver City.  A magnitude-3.5 earthquake hit that same area on April 12.

The Los Angeles quake struck while many residents were sleeping, and the timing of the Bay Area quakes Sunday was not lost on social media users.

Zachary Burkett tweeted, “It was nice of the northern California earthquake to show up in the mid afternoon instead of the stupid 4 AM jolt down here this morning.”

The Los Angeles Fire Department said in a statement early Sunday that it briefly went into “earthquake mode.” The alert was lifted after fire officials surveyed more than 470 square miles in the Los Angeles area and conducted safety checks.

COMPLETE QUAKE COVERAGE: CBS Earthquake Resource Center

DID YOU FEEL IT?: USGS Shake Map For Northern California
LIVE QUAKE MAP: Track Real-Time Hot Spots
BAY AREA FAULTS: Interactive Map Of Local Faults

Strong earthquakes with an epicenter off the coast can trigger tsunamis, depending on the size and type of the fault movement. The Pacific Tsunami Warning Center tracks earthquake data for the West Coast.
WEST COAST TSUNAMI TRACKING:

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