The San Andreas fault is a major transform boundary that runs through much of California, and it is justly famous for damaging earthquakes, including the 1906 San Francisco, the 1857 Fort Tejon, and the 1989 Loma Prieta events. As I've noted many times on this blog, it is not the only fault in the state, but it is the best known.
It's one thing to know the name of a fault, and quite another to know important facts about the fault, such as quake probability, potential quake magnitudes, and...where it actually is. The fault is not too hard to find south of San Francisco. The fault stays on land all the way to the Salton Sea region hundreds of miles to the south. North, though, the fault runs offshore, then briefly onshore, then offshore again. When it is onshore, the trace is often obscured at ground level because of thick vegetation, but the general location is usually easy to find on Google Earth and aerial photographs because of the straight linear valleys that tend to form along the fault system.
I was in the north bay over the weekend for purposes other than geology, believe it or not. But as I went through my photographs, I realized that I took a fair number of pictures of the San Andreas fault, except that it was invisible, being underwater and all. In the picture at the top of the post, we are standing at Muir Beach Overlook on the Marin Peninsula. If you click on the photo for the larger version, you can pick out Lands End and the residential buildings of San Francisco on the upper right part of the picture. The fault goes offshore out there, along the coast at Pacifica, and runs parallel to the coast to the right of the shot.