You don't have to rebuild the city every 100 years. From what I recall, most of the damage to the city in the 1906 earthquake was due to the fires which couldn'

You don't have to rebuild the city every 100 years. From what I recall, most of the damage to the city in the 1906 earthquake was due to the fires which couldn't be controlled, which was because the earthquake damaged the water lines too badly and they didn't have any working backups. But not only do we have better infrastructure now, we also have multiple auxiliary water systems put in place specifically for emergencies. If a big earthquake hit tomorrow there certainly would be a lot of damage, but there would not be widespread fire and the city would not be completely destroyed. Fun fact: One of those auxiliary systems is 177 cisterns containing between 75,000 and 200,000 gallons of water buried all over the city. If you've ever noticed red brick circles embedded in streets, those mark the cisterns. They're maintained specifically for dealing with fires.