In 1997 the building code was revised to require both stronger steel connectors and stronger seismic demands on any buildings near an active earthquake fault. Also part of the new regulations were higher seismic design forces for structures located on soft soils that can amplify an earthquake's ground motions up to 7.25 times what is experienced on bedrock. The new regulations took effect July 1. An older tilt-up suffered a partial collapse during the 1994 Northridge quake. A single point of ground can initially rise and fall as much as 3 feet while lurching back and forth horizontally up to 6 feet in a second or two.
Liquefaction occurs in loose, water-saturated sands as they are shaken by an earthquake.
The water portion of the equation is routinely provided by San Francisco Bay, which lifts the water table as close as 7 feet beneath a building's slab. The Association of Bay Area Governments expects vast areas of Silicon Valley to be susceptible to moderate to high levels of liquefaction during a major quake. Major liquefaction along Coyote Creek, which runs between north San Jose and Milpitas, was first recorded following the 1906 San Francisco earthquake, says Tom Holzer of the USGS in Menlo Park. Holzer says a second major problem for the Bay Area is the propensity of bay muds to amplify earthquake motions beyond what would be experienced on nearby bedrock. Amplifications of up to 3.5 times were measured during the Loma Prieta earthquake at Treasure Island.