Tuesday, November 18, 2014

"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!


The Field of Gold said...

The fault through the town. Is it a single line that can be traced all the way, though the place. Or is it a number of parallel cracks scattered about

Garry Hayes said...

I've only seen one active fracture over the last 25 years of visiting the site. There are two parallel faults mapped a quarter mile away on either side. In the park there is a small west-trending fault scarp, but I don't think it's currently active.

Check http://pubs.usgs.gov/bul/b2188/b2188ch6.pdf for a map of the fault through Hollister.

Anonymous said...

I attended college with a kid who lived in Hollister. He claimed that the fault ran through the middle of the family living room. One night, family sitting on one side of the fault said "Earthquake!". The other side said "Wha?"

I suppose it's possible. Funny, even if not true.