In 1906 there was very little “landfill” area. It has been built up greatly since the 1906 quake. Yes, water mains are vulnerable today. Also, the majority of the buildings in SF are not built to code - just after the 1906 quake new building codes were enacted - however, most buildings built after the quake until sometime during the 1950’s did not follow the more restrictive codes. When the next big one hits there’s going to be more devastation than that of the 1906 quake. And, given the state of our economic problems in CA, there’s going to be less ability to battle the resulting fires.
San Franscico will be a total complete disaster when the next one hits. The city fire department bought these two huge boats with the ability to pump water out of the the bay because the fear of broken water mains after a quake.