The reality of stress shadows is being seriously questioned. The quiet in NoCal after 1906 may well turn out just to be a random fluctuation, not an indication of lowered stress.

On the other hand, aftershocks are real, and a few can strike at a distance, as in Landers and the recent Alaskan earthquake.

Water wells often change their level after strong shaking. The best guess at the moment is that shaking clears out their pores, increasing permeability, which can shift the level either way.