Liquefaction in Past Earthquakes

Many of the liquefaction occurrences that have been caused by past earthquakes in the San Francisco Bay region have been mapped: see principally Youd and Hoose (1978, for earthquakes up to 1969) and Tinsley and others (1998, for 1989 Loma Prieta earthquake). The locations of these mapped occurrences are assembled in USGS Open-File Report 00-444, which includes a small-scale map (1:275,000 map of liquefaction susceptibility) showing the locations, together with a digital spatial database in which the locations and other relevant information about the occurrences are recorded.

The maps below show past liquefaction occurrences for two parts of the San Francisco Bay region, one covering the San Francisco and Oakland area (central Bay area), the other covering the south Bay area from Foster City to San Jose (southern Bay area). The backgrounds show liquefaction susceptibility from the present susceptibility map.

Liquefaction effects in the central Bay area from the 1906 San Francisco earthquake. (click to enlarge)	Liquefaction effects in the central Bay area from the 1989 Loma Prieta earthquake. (click to enlarge)
Liquefaction effects in the central Bay area from other earthquakes. (click to enlarge)	Liquefaction effects in the southern Bay area from all historical earthquakes. (click to enlarge)

Liquefaction in the 1906 San Francisco Earthquake

The magnitude 7.9 1906 San Francisco earthquake is the largest to have occurred in north central California since European settlers arrived. It is also about as large as future earthquakes are likely to be in the San Francisco Bay region. A significant fraction of the damage that occurred during and immediately following the 1906 earthquake was related to liquefaction, either directly or indirectly. During the earthquake, abundant damage was caused to buildings and structures by liquefied ground in areas like the Mission District and the Market Street area. The fires following the earthquake, which burned for several days, were so large and damaging in part because liquefied ground damaged the city's water system and severely limited the residents' ability to fight fires.

Pictured above: Filled land in San Francisco
(click to enlarge)

Much of the liquefaction-related damage in the 1906 earthquake occurred in "reclaimed" areas that were once bay or marshland. These areas of filled or "made" land were typically filled with pumped or dredged sediment from the Bay floor. This material was deposited behind a levee or dike and allowed to dry and settle prior to having structures built on it. Although the extent of filled or "made" land was not as great as it is now, there were still large areas of San Francisco that suffered from poorly engineered fills and liquefaction of these fills.

Liquefaction in the 1989 Loma Prieta Earthquake

The epicenter of the 1989 earthquake was in the Santa Cruz Mountains, distant from large Bay region population centers. It caused considerable damage to Santa Cruz because the earthquake was nearby. The more densely populated parts of the Bay region (such as Oakland and San Francisco) were exposed to relatively low levels of shaking, compared to shaking there from the 1906 earthquake and that predicted for future earthquakes near the core of the region. See a comparison of shaking levels from the two earthquakes -- wait for the two maps to alternate (Boatwright and Bundock, 2005).