I think the soil has something to do with it. In the 06 earthquake, there was more actual quake damage in Santa Rosa than in San Francisco, it was the fire that got SF.
The 69 quake here in Santa Rosa was a pretty wild ride as well. Was sitting in my folks backyard and the ground started rolling with waves that looked to be about a foot high. (They were probably much less than that but it sure looked like that high) Trees swaying back and forth, crap in the house falling off the walls, etc.
Stan