This is from memory, and I have not been able to confirm it with a cursory search, but FWIW: In most of central London, the bedrock is far below (which allows for deep Tube tunnels), so high-rises do not rest on bedrock, and are at least partially supported by the buoyancy of large rigid basements.

Of course, London does not have the seismic risk of San Francisco. I would guess that buoyancy might be an asset with regard to support, but possibly a liability with regard to stability, in the face of liquefaction.