The U.S. Geological Survey (USGS) is releasing two new digital maps of the densely populated central part of the San Francisco Bay region. These maps are designed to give the first responders, as well as the general public, those responsible for maintaining utilities and other “lifelines,” public officials, emergency response personnel new and better tools to assess risk from earthquake shaking damage. The maps also serve as the baseline data for the California Geological Survey’s Seismic Hazard Zone maps.
Two new maps give first responders, land use planners, decision makers and Bay Area residents a new and more detailed look at the risk of liquefaction in the soils underlaying buildings and other important components of the Bay Area infrastructure, such as roads and pipelines.
The first of these products is a new map of the young geologic deposits in the low-lying sections of the Bay Area. Most residents of the Bay Area live and work in these low-lying areas that are underlain by these young deposits. These deposits are important because so much of our infrastructure resides on them, and because they can host severe earthquake effects. Some can undergo liquefaction, the phenomenon in which saturated soils lose their stiffness and strength during shaking. And some can greatly increase the severity of earthquake shaking that is transmitted through the deposits.
The second of the map products, derived from the first map, shows the likelihood that these young deposits will liquefy, or turn into a sandy liquid due to the strong shaking a big earthquake will produce. When the ground liquefies, it may lose its ability to support buildings and other structures. Liquefaction during large earthquakes commonly disrupts pipelines and road networks and also may cause buildings to settle and move down very gentle slopes or toward stream banks.
The highest hazard areas shown by the liquefaction hazard maps are concentrated in regions of man-made landfill, especially fill that was placed many decades ago in areas that were once tidal flats and submerged bay floor. Such areas along the Bay margins are found in San Francisco, Oakland and Alameda Island, as well as other places around the Bay. Other potentially hazardous areas include those along some of the larger streams, which produce loose young soils that are particularly susceptible to liquefaction.
Then new map products are the latest in a series of new tools and events that showcase the lessons learned with the 100th anniversary of the 1906 San Francisco earthquake fast approaching.