Over the years, I’ve written up a few side trips from various BART stations (El Cerrito, Lafayette and Warm Springs here and Hayward, 16th/Mission and the phantom Irvington station elsewhere). Time for a look at Concord, where one of our major faults runs right through town.
At our latitude, the Concord fault marks the inner edge of the wide boundary zone between the Pacific and North America tectonic plates. The two plates rub past each other at a total rate of about 50 millimeters a year. About half of that motion takes place on the San Andreas fault and smaller fractions on the Hayward, San Gregorio, Calaveras and other faults. The Concord fault is closely connected with the Green Valley fault across Suisun Bay. They may rupture together.
Location map modified from Sharp (1973)
Concord began existence as a farming village perched on a low rise overlooking the wide, fertile valley of Walnut Creek-the-creek. The town square and the Galindo Adobe mark the spot. At the lower edge of that rise is the Concord fault, which nobody knew because it hasn’t ruptured the surface in written history. Farther south the fault runs along the foot of Lime Ridge, where I stood to take these photos looking north over Concord/Pacheco and south over the Ygnacio Valley, respectively.
Like Oakland, Concord expanded across its earthquake fault before they knew it had one. Today we have a fair idea of where the fault is located, and the state has declared the fault an official Alquist-Priolo hazard zone. Here’s the part near central Concord, where the fault is most confidently mapped.
From the state’s EQZApp server
Like some other Bay area faults, the Concord fault exhibits creep — slow, ongoing surface movement without earthquakes. This allows us to map it without waiting for a big quake by seeing where creep disrupts the ground. The map also shows three spots to see the fault within an easy walk of the Concord BART station, which is just east of locations 1 and 2.
Location 1 is the Laguna, a geomorphic feature of the fault. This is the view south from Laguna Street.
It’s a notable landmark, originally known as La Laguna de los Balbones and Galindo Lake later. Ruth Galindo, the last survivor of Concord’s old Mexican landowning families, told the local paper in 1960, “ask any old resident of Concord and he or she will remember the Laguna.” The natural pond suffered as wells around it depleted the groundwater. It became part of the Keller Ranch in the 1930s and was called Keller Lake. In the early 1960s it fell into the hands of apartment developers, and now this interesting bit of history is off limits to the public, although people do fish from the armored shore on Laguna Street.
Geologists recognize the Laguna as a sag basin, a type of depression that’s common along California’s active faults. They form where two branches of a fault, because of their geometry, spread the ground apart between them. (The opposite kind of landform is a pressure ridge.)
Location 2 is 1688 and 1689 Ashbury Street, where the fault has warped the road over the years. Here are views east and west, respectively, showing the bent curbs and amateur concrete patches.
Other much more subtle examples of offset curbs can be seen in the blocks just north of location 3. There’s also this offset at the north end of Stanwell Drive, at the top of the map.
Location 3 is at the west end of Salvio Street. Walking north into the Dollar Tree’s parking lot, you’ll find this pretty set of en echelon cracks marking the fault . . .
. . . at least until the lot is resurfaced. The sidewalks and street here were recently rebuilt, erasing any signs of creep for the next few years. The cracks continue up the parking lot . . .
. . . and end at this groundwater seep.
Active faults typically are barriers to groundwater, forcing it to rise toward the surface and even above. Little springs are common along our active faults, especially in the rainy season.
There are probably other similar features at the south end of the map, east of San Miguel Road and the BART tracks, but that’s a large mobile-home park where I didn’t feel like intruding.
Oakland Geology 
6 February 2024 at 4:27 pm
I’m unclear how to subscribe to the Oakland Geology email list. I want to recommend it to someone.
[Look for a little popup in the lower right corner. — Andrew]
7 February 2024 at 9:11 am
Interesting. I just read what I thought was a convincing argument for the western “edge” of the NA plate being somewhere east of the Sierra Nevada, maybe in the Walker Lane area (Meldahl 2011, Rough-hewn Land). So perhaps his scenario is not widely accepted … ?
[Most of California between here and the Walker Lane is considered the Sierra microplate. It moves very slowly with respect to the rest of North America, a millimeter or two per year. Meldahl’s books are very good. — Andrew]