Correspondent Brian Rooney blogs about his recent trip to San Francisco: It’s a funny thing when you walk around in a city. You don’t think much about what used to be there. I’ve been going to San Francisco for thirty years and never gave much thought to the ground it was built on. That’s not the same as knowing that the San Andreas Fault runs right past it. I spent a couple of days in San Francisco last week shooting a story on the anniversary of the great earthquake, something else I had never given much thought to. I had no idea that by far most of the damage was done by fires that ran uncontrolled for three days after the quake. The gas and water mains were severed so buildings caught fire and the fire department couldn’t put it out. Finally the military dynamited 25 blocks and stopped the fire at Van Ness, the city’s widest street at the time. But still, they lost 28,000 buildings. (At left, several thousand people gather in San Francisco’s financial district around 5:12 a.m. this morning to commemorate the 100th anniversary of the 1906 San Francisco earthquake. The magnitude-7.8 earthquake struck at 5:12 a.m. on April 18, 1906.) You don’t think about it now, or at least I didn’t, but San Francisco was on its own at the time. There was no real help to be had from anywhere else. Los Angeles was little more than a village. And I didn’t know that San Francisco rebuilt quickly, and on even worse ground than before. They built on top of filled-in creeks and bogs, even filled in the little lake where the original mission had been established. They expanded the city out onto sand dunes and areas that are bay sediment. They plowed earthquake wreckage and sand into the bay, and built on top of that, too. Major portions of San Francisco … The Marina, for instance …. Are built on top of ground that would turn to Jell-O in a big quake. Another fun fact: the fire department has two hydrant systems, high pressure and regular. They also have underground vaults of water to tap into if water mains are broken.