The mapped geologic units in the Bay Area can be grouped according to an approximate material shear strength classification of A, B, or C, with A being those units least susceptible to sliding and C being those units most susceptible to sliding. The final factor included in this analysis is degree of water saturation. This variable depends in large part on length of time since the last major storm and rainfall to date. The intensities required for landslides tend to be lowered by approximately one intensity unit under wet conditions. Currently, this type of Seismic Hazard Zone Map is only available for San Francisco, Oakland, part of San Jose and several areas in Los Angeles, Ventura and Orange counties in southern California.
Additional mapping is subject to the availability of state and federal funding.
Second, vegetative cover is not incorporated into the following tables because very little research has been conducted quantifying its effect. When the ground liquefies, sandy materials which are saturated with water can behave like a liquid, instead of like solid ground. In essence, the sand grains momentarily behave like a liquid. The ground needs to be shaken strongly for liquefaction to occur, and this shaking can occur as a result of an earthquake. Liquefaction can cause ground displacement and ground failure.