Is there anyone in the Bay Area who hasn't heard of soil liquefaction from seismic activity? That's when an area isn't on solid ground and the force of an earthquake makes it slosh around like ball bearings in a cat box. It happens especially in land fill areas.
Since the Gold Rush, people have been filling in the tidal mud flats with dirt, trash, cement blocks, and anything else they could find to make "solid" land out of mud.
Wikipedia has some good info on it liquefaction, and here's the picture from that article.
Okay, we all know it's all over the place but where is it exactly?
Well, I can't tell you exactly but the state of California provides a map that gives us a pretty good idea. Check it out here, and here's the actual address in case you want to cut and paste it. http://gis.abag.ca.gov/website/Hazards/?hlyr=liqSusceptibility
And possibly either alarming or reassuring, depending on where you life.
(If you live in the liquefaction zone, earthquake insurance might be an especially good idea.)