San Francisco has been built by the grace of the grace of the 1906 quake. When the big quakes happen, they release all kinds of stress, and make a future local

San Francisco has been built by the grace of the grace of the 1906 quake. When the big quakes happen, they release all kinds of stress, and make a future local quake much more unlikely. It's called the stress shadow. The stress shadow of the 1906 quake was (depending on who you ask) about 100 years. SF is coming out of the stress shadow, and is become "due" for a new big one. WRT the rest of the valley, the main problem is that it's built on sedimentary rock. Sedimentary rock slows the seismic waves, which since you need to conserve energy, results in larger shaking. In other words, a small quake further south could result in more damage in the valley.