Why is San Fransisco vulnerable to earthquakes?

I actually lived in Cali so maybe this will help. My mother and grandfather said it’s because it’s basically “floating” on the water and the water will (kindove) move the island/bridges. It might also be because California has earthquakes all the time near there so it is more destructive. It could also be that the buildings are so cramped(believe me,their relly cramped) that it could cause a chain reaction as all the buildings are basically connected