Getting a Creepy Feeling in Central California: A "Transforming" Experience

Faults can be active without producing major earthquakes. When the fault plane is not locked by frictional resistance, the sides of the fault can slide past each other without building up the stress that leads up to huge quakes. In Central California, there are several faults creeping on a more or less consistent basis year after year. They are close enough to each other that they can be easily visited in an afternoon. That is exactly what we were doing last Saturday.

The town of Hollister is bisected by the Calaveras fault, and numerous homes, sidewalks, and streets are slowly being torn apart by the constant movement, amounting to as much as 14 mm/year (0.55 inches). The embankment in the photo above appears in a number of geology textbooks as an example of fault motion.

The foundation for this 1920s vintage garage has also been seriously damaged. We've been visiting the spot for 28 years. It will be disappointing when they finally decide to renovate the building!

Probably the most famous example of fault creep in the region can be found at the DeRose Winery a few miles south of Hollister. The fault is far more active, with yearly offsets amounting to as much as 25 mm/year (1.0 inches). One can get a sense of the magnitude of the motion by comparing Saturday's image to one taken half a century ago (below) 

Source: NOAA/NGDC, University of California, Berkeley.

The DeRose Winery has changed hands a number of times in the last 160 years, and the owners of the place in the 1980s kind of let it go (I believe they were even trying to produce tiger prawns in the old wine vats for time). The current owners are a treasure. They recognize the scientific value of their property, and have been clearing weeds from the drainage that threatened to completely hide the iconic symbol of right lateral strike-slip motion. They have also never refused us entry into the warehouse where offsets accumulating since 1948 are nicely exposed in the walls, even when formal wine-tasting has been going on.

All of these incredible exposures are easily accessed. If you are ever in the vicinity of Hollister or Pinnacles National Park, don't miss the opportunity to lay your hands on a major transform plate boundary!

"A City Divided!" "Rift Tears City Apart!" "Slip-Sliding Away!" "Creepiness on the Calaveras"... I Give up: "Calaveras Fault in Hollister"

Blogging has certainly given me some perspective on life at newspapers or internet news services. The title of a piece is all you've got to convince people to click through. I am constantly irritated beyond words at the clickbait out there: "She thought no one was looking. You won't believe what happened next". That kind of thing. And yet I couldn't help it. How many ways can you say a fault can be seen cutting a city in half?

If you have ever had a geology class in the Bay Area or Central Valley of California, there is a chance you went on a field trip to see the Calaveras fault where it passes through the Coast Range town of Hollister. The Calaveras, a close cousin to the better known San Andreas fault, runs through the city, and it is slowly tearing neighborhoods apart. Like the San Andreas in this region, the fault is creeping instead of storing up seismic stress, moving perhaps a quarter inch per year. Curbs, streets and foundations are all slowly cracking and moving apart.

The creep has progressed to the point of seriously damaging some of the beautiful Victorian-style homes built in the 1920s and 1930s. Several have been repaired by lifting them up and replacing the foundation beneath them.

Street and curb repairs have been ongoing, with new patches showing up on a constant basis.

These are textbook examples of the effects of fault creep. Literally. Pictures of many of these homes and sidewalks show up in geology textbooks (including one or two of my pictures) as examples of the phenomenon.

The neighborhood has been the focus of hundreds of field trip visits over the years. I would hope that the students recognize that people live in this neighborhood, and thus respect their privacy and property. There is more than enough to see from the public sidewalks, streets and parks. 

No trip in the region is complete without a visit to the DeRose Winery, especially on wine-tasting day (and no, none of my students imbibed on Saturday)! The winery, the oldest in California, is several miles south of Hollister on Cienega Road. The San Andreas fault runs right through the winery warehouse and is slowly tearing the building apart. A culvert on the south side of the building shows very well the continuing offset along the fault.

Compare the same culvert as seen in 1961...

Source: USGS

I want to provide a shout-out for the owners of the DeRose Winery. Whenever they've been open on Saturdays they've allowed our students to walk through the inside of the warehouse to see the effects of faulting on the building. It's a privilege to be able to do this, and it is greatly appreciated. Follow the links above to learn more about their operation.

If your travels ever bring you to central California in region around Monterey or Santa Cruz, it's not hard to find your way to Hollister. It's worth your time!

Do Real Estate Agents Have to Tell You About This Kind of Thing? California Has Her Faults

It was the third weekend in November, and thus time for our last field studies excursion of the year. We headed into the Coast Ranges of California, braving rain and wind for a chance to investigate the San Andreas and related faults, and to explore one of the lesser-known gems of the National Park System, Pinnacles National Monument. One of our first stops (and indeed a stop for practically every field class that ventures forth into Central California) is the town of Hollister, south of San Jose, and sitting right on top of the Calaveras fault.

The Calaveras fault is an offshoot of the San Andreas, separating from the main fault a few miles south of town. The Calaveras is quite clearly active, as can be seen in these photos. The fault is well-known for creeping rather than sticking and slipping (and producing ruinous earthquakes).

The fault moves at rates as high as half an inch per year, although the rate is highly variable. The fault runs through numerous homes, and the movement is slowly tearing the foundations apart. Every decade or so homeowners must lift the houses up and replace the foundations.

The side of the porch has shifted to the right, away from the steps

The fault is easily traced in the neighborhoods between 1st Street and 6th Street at Dunne Park. I've noticed that folks living on the trace of the fault barely look up from their porches as classes walk down the street following the fault zone. A few are proud enough of their notoriety that they will come and describe what they've been doing with their faulty houses.

Our trip route took us south of town, to where Cienega Road intersects and then follows the San Andreas fault. Beyond the Hollister Hills State Recreation Area, one reaches the DeRose Winery, another famous fault locality. The winery was built directly on the trace of the fault, and has been destroyed twice by creep along the San Andreas.

The owners have always been kind to our students, allowing us to sneak through the wine-tasting event to have a look at the interior walls, which have been offset by several feet (below).

 The wood post and the cement slab started out together...

The water-course at the south end of the winery is one of the most famous drainage culverts in existence because of how it has been offset by the fault (I forgot to get a picture this year; this is last year's version).

I don't know if realtors have to disclose the fault problems of home-buyers in Hollister, but if the DeRosa owners ever have to sell their winery building, they'll have a hard time hiding their fault...there is a brass plaque in the middle of the building declaring it a Registered Natural Landmark by the Department of the Interior!

The other San Andreas fault locality for our trip is on Highway 25 where the road is traversed by the fault. The road was completely repaved in the last year or two, but if you look carefully, you can see that the fault is already at work tearing the road apart again.

 For a sense of comparison, below is a shot from 2003...

 ...and a shot from 2008 after a patch job.

We had an audience at our stop...a gang or rafter of dinosaur descendants trying to avoid being the main course later this week!