The Airliner Chronicles: The San Francisco Peninsula and the San Andreas Fault

Crystal Springs Reservoir is on the upper left, while San Andreas Reservoir is on the lower right

As has been no doubt obvious, I was in Hawai'i last week, and there have already been several posts about some of my adventures. I started through the many pictures in a more chronological manner, and realized there were some neat things I saw before I even left California. I always notice on a flight that some people do such things all the time and are not particularly impressed that they are in a large metal tube traveling at hundreds of miles an hour at 35,000 feet. Others have a sense of history, realizing that for the first few million years of hominid existence, this was an unnatural thing to be doing. I don't get to fly all that often, so I tend to fall into that second category. As soon as we are off the ground, the camera that I surreptitiously placed at my feet swings into place and I start snapping pictures as if I were the first human being ever to see such incredible sights. My photo habit led to my first blog series in 2008, the Airliner Chronicles. Consider this a new entry...

It is the geologist's perspective that provides some insight as to the nature of the landscape below. We left from Oakland Airport and flew over the north end of the San Francisco Peninsula, which provided an outstanding view of the San Andreas fault. Often such faults are indicated by linear valleys owing to the ease with which crushed rock in the fault zone erodes. But on the peninsula the fault is even easier to pick out because of the presence of two reservoirs filling the linear valley, Crystal Springs reservoir, and San Andreas reservoir.

It's quite a coincidence, the fault and the lake both being called "San Andreas", so the question naturally arises, which was named first? The fault may be more famous to most people, but the lake came first. It was constructed in 1868. In 1895, the geologist Andrew Lawson was mapping in the area and discovered the fault, which he named after the lake. He had no idea that the fault was 600 miles long, or that it was particularly active, or that it was the boundary between the Pacific and North American tectonic plates. And he most certainly didn't know that it would shift in 1906 in a most tragic way.

The city has grown in the years since 1906, to say the least. We know a great deal more about fault zones and earthquakes, and thus we have a better understanding about the threats of future quakes in the Bay Area. Both by law and by subsequent experience (Loma Prieta in 1989 and the Napa Valley in 2014), the cities of the peninsula are better prepared for large earthquakes, but no matter how ready they may be, the next large earthquake will cause major damage on the peninsula and many will die or be injured. Seeing the proximity of the fault and the cities from above provides a stark reminder of the need to be prepared.

We flew out over Half Moon Bay, and soon land was left behind. There was more than 2,000 miles of open ocean ahead of us, a five hour flight. I settled in and thought of how the islands were inaccessible to humans of any kind until only a thousand or so years ago, and even a hundred years ago it took weeks or months to cross the ocean. How even more wondrous the number of bird and insect species that survived the journey over the millennia.

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...