The Return of the Airliner Chronicles! The San Andreas Fault in San Francisco

I love flying. Well, I don't like airports. Or security lines. Or airliner food. Or uncontrollably crying babies (though I have sympathy for the poor parents sometimes). Or the incredibly small seats. Or the lack of foot room. Or looking for lost baggage. Or airport shuttles. I don't like all of that stuff.

But put me in a window seat on a clear day, and a route over a geologically interesting place, and I will be deliriously happy. I will easily snap dozens of pictures when conditions are right. The Airliner Chronicles was my first idea for a blog series, and I abandoned it for a few years only because I haven't flown anywhere since 2009. But I flew this holiday vacation, so I am pleased to return to the Chronicles once again.

I usually fly out of San Francisco, but it has been rare that I have had a clear view of the San Andreas fault in the Bay Area. On this flight, I almost felt the pilot knew I was on the left side, and that I wanted as many sight angles on the fault as possible, so he or she banked us nicely for four perspectives of the San Andreas fault as it runs from the hills above San Jose to the sea at Daly City near San Francisco.

Have you ever wondered how the San Andreas fault got its name? In 1895, U.C. Berkeley professor Andrew C. Lawson mapped a fault in the Bay Area that crossed San Andreas Lake/Reservoir (which is at the bottom of the first three photos; the other lake is Crystal Springs Reservoir). The 1906 San Francisco Earthquake made it clear that the fault was a much larger feature (200 miles fractured in 1906, but the fault extends some 600 miles to the Salton Sea area in Southern California).

The dam that holds back San Andreas Reservoir was damaged in the 1906 event, but the dam held. The west side of the fault (right side in the photo above) lurched about 9 feet northwest during the quake. Faults that move sideways like this are called strike-slip faults, and if the side of the fault opposite the observer moves to the right, it is a right-lateral strike-slip fault. The San Andreas is this type of fault.

Even more importantly, the San Andreas fault is a transform boundary, marking the spot where the Pacific Plate is moving northwest relative to the North American Plate (or the Sierra Nevada Microplate if you want to be really specific). The fault has been moving for nearly 30 million years, and has shifted 200 miles or more (roughly 2 inches per year). It would be more correct to say that the fault would shift 2 inches per year, but friction holds the sides of the fault in place until enough stress has built up (a century or two) that the fault breaks and moves 10-20 feet all at once in a large earthquake like the one in 1906.

We banked one more time and had a wonderful look at the entire San Francisco Peninsula before heading east towards the Sierra Nevada...

Hey! It's not my fault! It's San Andreas' fault...A Creepy Story

DeRose Winery drainage culvert in 1961 (courtesy of USGS field guide)

Quick, what's the difference between the pictures above and below? Well, OK, besides the growth of vegetation and the passage of 50 years?

DeRose Winery drainage culvert yesterday, courtesy of me...

If that drainage culvert looks like it has moved several feet in the last fifty years, you would be right. It was built right over the San Andreas fault, and in this part of California the San Andreas creeps a little bit each year (this is a good thing; this part of the fault doesn't build up stress, so it doesn't produce large earthquakes).

The drainage culvert is a famous locality that is visited by hundreds of geology students every year. It is located at a winery about eight miles south of the town of Hollister in the central California Coast Ranges. That's what we were up to yesterday, exploring the San Andreas and Calaveras fault zones with my geology students.

As I recall the story, the winery was first constructed in the 1800's, but for some inexplicable reason, it fell apart. It was rebuilt and then fell apart again, and then was constructed once again at the present location in 1948. When the building started showing signs of stress again, an investigation in the late 1950s led to the conclusion that it had been built on the San Andreas fault, and that the fault was moving around 0.5 inches per year.

As can be seen in the picture above, the building is suffering serious damage again. We have been coming here on field trips for twenty years, and something different can be seen every year as new patches, repairs, and repaved roads can be seen. But in all those years, there was something we had never done before...we had never gone inside. By a quirk of scheduling, we arrived earlier in the day, and stumbled onto a wine tasting event. We headed inside to have a look around (note to my dean: we did not stumble out...no one went wine tasting in our group!). The damage inside actually looked worse than what we could see on the exterior.

It's not like they don't know the building is on the San Andreas...that bronze plaque on the wall in the picture above is a 1965 designation of the site as a Registered Natural Landmark, and there is a map of the San Andreas fault in the poster to the right.

The wood post on the left was once even with the concrete wall on the right. The concrete has moved to the right, making the San Andreas fault a right lateral strike slip fault. I've outlined the sense of motion in the picture below.

I'm wondering if any of the structural geologists out there can make sense of the fracture patterns of the the concrete on the floor in the picture below?  It looks to me like there might be some conjugate fractures and rotation among some of the chunks.  I appreciate that the owners seem not to have fixed that part of the floor (I wonder if that was a stipulation of being declared a national landmark?)

The north wall has some serious warping problems (below). The doorway is nearly impassable as the wall in the foreground is shifting more and more to the left.

I've written about this trip a few times. The point where the San Andreas fault crosses Highway 25 that showed clear signs of fracturing on our trip last year (blogged here) has been repaved, so we'll keep an eye on it for changes over the next few years.

It's easier than you might think to see the San Andreas and Calaveras faults for yourself. There are several good road guides in books and online. An excellent one by the U.S. Geological Survey can be accessed here:  http://pubs.usgs.gov/bul/b2188/b2188ch6.pdf

There were some pretty sights at Pinnacles National Monument. More in the next post!