Cathyryn (I read Mike's comment)..
To get a M8 the quake needs to rupture through many segments the length of Loma Prieta. Sometimes the rupture keeps on going, sometimes it stops. You need it to break 300 km or more to get a M8 on a strike-slip fault. The Loma Prieta segment was part of the 1906 quake and also broke in 1989. It is possible that the part near and north of San Francisco tends to keep on going once it starts, but I would not rule out a M7 (or 6..) on any part of this...I would say that what makes quakes stop is not really well understood (although breaks in the fault and lack of accumulated stress can do it).
Chris...in Santa Barbara