[monkburger]

More of a concern in the bay area, the geological composition is sand, mud, basic river delta sediments--not the most stable stuff.

eg: During a hypothetical Earthquake in the Bay area, the liquefaction of sand occurs, which causes underground structures to move upward due to buoyancy (think of a lower level parking garage), and the soil mass from the structure collapses.

the liquefied sand diffuses to the surrounding area after the earthquake, causing the sand beneath the underground structure to be lost, and the building will return to its original position or even collapse.

[inference-lord]

Yeah, also quite bad, I've seen the aftermath of this liquefaction in Japan. Whole apartment blocks topped over with seemingly good foundations.