In Fremont, construction is underway on the Warm Springs Extension of the BART light-rail system. Two new stations are in the plans, the first one being the "optional" Irvington Station at the intersection of Washington Boulevard and Osgood Road. It will be right next to the century-old ruins of the Gallegos Winery, where cultural history coincides, as it so often does in the Bay Area, with geological history.
What interests the geologist about the extension is that the rail line will cross the Hayward fault in two more places (BART already crosses the fault inside the tunnel between the Rockridge and Orinda stations). The Fremont Station, at the end of the line today, is just short of the fault. As you look south from the station, the fault comes from behind to the left and crosses to the right just across Walnut Avenue. Tule Pond, the little lake to your left, sits in a sag basin on the fault (learn more about them on the San Andreas Fault Trail across the Bay at Los Trancos Preserve).
The new track will cross the fault leftward, dive underground beneath Fremont's Central Park and re-emerge just north of the Irvington Station headed due south. It will cross the fault again, rightward this time, about where Washington Boulevard and Osgood Road meet. Here's the fault superimposed on a Google Maps image of the area.
The low hill on the east side of Osgood is most probably a pressure ridge, the opposite of a sag basin, that marks the fault just like the pressure ridge in Hayward. Here's how it looks a little south of the station site; the Gallegos Winery was dug into it.
The Hayward fault was attractive to builders before anyone understood faults. One reason is that faults often bring water to the surface; the other is that faults often build useful elevations in the ground. In Irvington, Juan Gallegos built a large state-of-the-art winemaking plant here in 1884 so that grapes could be loaded from the high ground in back and processed with the help of gravity. A rail spur served the front side, carrying thousands of barrels of excellent East Bay zinfandel to a ready market.
The three-story brick structure was a landmark much like its counterparts in Napa today, surrounded by gardens and palm trees. The building was destroyed by the 1906 earthquake (107 years ago today), but the land remained a garden estate, having been subdivided long before when phylloxera and a poor economy ended the good times.
Much later, the Hayward fault was mapped through here, and the abandoned site lent itself to scientific monitoring. One of the fault's four creepmeter installations is here. A creepmeter is a rod of quartz or Invar metal, securely mounted and housed in an underground climate-controlled vault that extends across the fault (see the plans here). It's regularly measured to track the slow, steady fault movement known as creep.
The creepmeter shows the fault here moving between 6 and 7 millimeters a year. The wall at the right side of the photo is also being bent by creep. Ideally, when the Irvington Station is built, this site will be spruced up and some interpretive signage put in. Fremont, unlike most cities along the Hayward fault, has done a lot to inform the public about it.
Once upon a time, Fremont was a celebrated wine district. It was good land, watered by thrust-faulted hills. If not for one or two accidents of history, today Fremont might be as prominent as Livermore and Napa as a wine region. Indeed, the whole East Bay could have made a superb viticultural region if the timing of its earthquakes had been a little different. Something to ponder with your aperitif.