The water mains in San Francisco are still vunerable to breaking today from a major quake. The other reason most of the city was destroyed is because of landfill.
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.