3.6 Quake in Contra Costa County Shakes Bay Area

Interactive U.S. Geological Survey map showing location of Sunday's Concord quakes. Click on dots for quake details.

Update, 10 p.m.: The U.S. Geological Survey has reported two more small shocks in the same area as this afternoon's earthquake: a 2.3 tremor at 5:54 p.m. and a 1.5 shakelet at 8:30 p.m. That's a total of eight small quakes in a Concord neighborhood southeast of Buchanan Field since the first one at 7:15 this morning.

Update, 6 p.m.: The U.S. Geological Survey has revised downward its report on this afternoon's quakes on the Concord Fault, now saying the largest in the series of five shakes was 3.6 instead of 4.0.

Original post: We'll admit right off, from KQED Weekend Blog Central in beautiful Berkeley, that we did not feel the 4.03.6-magnitude quake that shook parts of the Bay Area at 3:13 p.m. Sunday. Neither did we feel the 2.7 mini-temblor a minute later.

Nor the three other episodes of trembling earth the U.S. Geological Survey reports happened between 3 p.m. and 4 p.m. All five shakes were centered in a tight cluster a mile and a half or so southeast of Concord's Buchanan Field airport.

Neither injuries nor damage have been reported.

Our apparent quake insensitivity notwithstanding, the USGS "Did You Feel It?" page recorded thousands of responses from San Francisco to Stockton and from the Santa Cruz Mountains all the way north to the Napa Valley. But even close to the epicenter, those responding described the quake's intensity as "weak" to "light."

The USGS summary of the largest quake places it on the Concord fault. The summary reports that the rate of movement, or "creep," on the fault is about one-sixth of an inch per year. Also, the USGS says, the best current guess is that the fault last produced a major earthquake two to five centuries ago.

As the Contra Costa Times pointed out last month, the fault and its potential to generate a big quake is of special concern now because of some of the amenities of modern life -- big oil refineries -- that have been built nearby:

What makes the Concord Fault particularly worrisome to regional planners, so much so that it was highlighted in a December study by the Association of Bay Area Governments, is its potential impact on regional and statewide fuel distribution. Without gasoline, every other crucial need, including water, electricity and transportation, will be affected.

In its report, ABAG studied three theoretical earthquakes -- a 7.9 on the San Andreas Fault, a 7.0 on the Hayward and 6.8 on the Concord.

"Originally, we were just going to explore the San Andreas and Hayward faults, but we realized that (there are) a lot of key infrastructure assets in (the Concord) region," said study author Michael Germeraad, an ABAG resilience planner.

Five Bay Area refineries -- all but two are within a couple miles of the fault -- processed 235 million barrels of crude in 2012, about 40 percent of the state's total, according to ABAG. In addition, Kinder Morgan operates a pumping station nearby that receives processed crude from all the refineries and pipes it out to terminals across Northern California and Nevada.