Reminder: A magnitude M earthquake is about 10 times less likely than a magnitude M-1 earthquake (the Gutenberg-Richter Law).
The San Francisco earthquake of 1906 was Mw = 7.8. The 1989 Loma Prieta earthquake in the same region was Mw = 6.9. The Boxing Day earthquake of 2004 in Indonesia, which caused the huge tsunami, was Mw=9.3.
The USGS estimates that, since 1900, there have been an average of 18 major earthquakes (magnitude 7.0-7.9) and one great earthquake (magnitude 8.0 or greater) per year, and that this average has been relatively stable.
UPDATE: A tsunami warning has been released for the surrounding area, all the way to New Zealand. (That doesn't necessarily mean a tsunami has occurred.)
NOAA's Pacific Tsunami Warning Center is swamped right now (11:50 am PDT) -- just when the world needs it most. You'd think they'd have a contingency plan for exactly this situation.