If there's an earthquake on this fault, how bad would it be? Would it trigger a tsunami? Would SF and LA be damaged?
Could this cause "the big one" that gets mentioned occasionally? How bad would such a quake be?
These generally do not cause as much damage as a subduction zone event, also known as dip-slip. An example of this type of event (dip-slip) would be the 2004 Indian Ocean earthquake, or the violent 1700 Cascadia Megathrust Earthquake.
Personally, Cascadia is /more/ dangerous than any strike-slip earthquakes like SA. We have sedimentation records of massive Cascadia tsunamis going back to about ~900CE.
I know that seems like a non-issue in the grand scale of things, but Tsunami's of the past didn't do this. Most pollution was biodegradable.
I can't imagine what the coast of LA would look like after a similar event :(
eg: During a hypothetical Earthquake in the Bay area, the liquefaction of sand occurs, which causes underground structures to move upward due to buoyancy (think of a lower level parking garage), and the soil mass from the structure collapses.
the liquefied sand diffuses to the surrounding area after the earthquake, causing the sand beneath the underground structure to be lost, and the building will return to its original position or even collapse.
Earthquakes' effects are most strongly felt immediately around the epicenter, and diminishes with the square of the distance.
The San Andreas Fault, being a strike-slip fault (plates moving laterally against one another) and lying principally inland has very low tsunami risk. Tsunami are most associated with superthrust faults, in which one plate rides up over another, and energy is release with a strong vertical displacement, as with the 2011 Tohoku and 2004 Indian Ocean earthquakes.