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.
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| 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!
4 comments:
Wonderful photos, Garry. Thanks for such clear examples of how the ground is creeping - must feel a bit weird for the residents, surely?!
Under the Alquist-Priolo act, it is necesssary in California to inform potential property buyers of hazards like faults, whether the property is located in a wildland fire interface zone, etc. I suspect it's really hard to sell one of those houses.
Those patches in the road, 11th picture down, look suspiciously like en echelon fractures.
Although I hadn't had much geoscience education at the time, both husband and I were educated enough (thanks to an undergrad intro Geology course) to get a fault map and make sure our house wasn't on any faults before we bought it. Since then we gathered that we're rather unique in that regard. How sad; some of this stuff should be taught in high school, at least in California!
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