What difference does the type of soil make?

“My understanding is that most, if not all, of the skyscrapers in San Francisco are supported by piers/piles that are drilled through the soft soil and anchored into the bedrock 300 feet below street level.” — T Roll, San Francisco

Soft soils amplify ground shaking, which is one reason to build on rock. Parts of downtown San Francisco, especially the South of Market area, used to be part of the Bay. The sand and clay used to fill that area comes with the risk of liquefaction, in which the ground becomes quicksand after an earthquake. Some skyscrapers, like Salesforce Tower, are built in areas of soft soils but with construction piles extending to the bedrock deep beneath the surface. This strengthens the building, but the area around it could still suffer significant damage from liquefaction and shaking.

Could there be a domino effect?

“Could an earthquake trigger a domino-type situation with one toppling skyscraper knocking into others?” — Margaux, California

Here is Ron Hamburger, one of the country’s leading structural engineers: “Very few of them should fall down,” he said of skyscrapers. “We would expect that most of these buildings — 60 or 70 percent — would be repairable.”

California’s building codes call for a 90 percent chance that a structure avoid total collapse. But many engineers and earthquake experts add a crucial caveat. Our newest high-rises are designed with computer models that have never been tested by a very large earthquake. And with each major earthquake we learn more about the violent ground motions that accompany them.