Laserfiche WebLink
<br /> <br />Additionally, Figure 6 is a map of Liquefaction Hazard Zone of Required Investigation for <br />discrete portions of the Bay Area (Alameda, San Francisco, and Santa Clara Counties). This <br />map is produced by CGS as part of its mapping program mandated by the Seismic Hazards <br />Mapping Act. The CGS liquefaction zone maps are based on the presence of shallow historic <br />groundwater in uncompacted sands and silts deposited during the last 15,000 years and <br />sufficiently strong levels of earthquake shaking expected during the next 50 years.13 Like <br />the fault zone maps, these official seismic hazard map zones require real estate disclosure <br />upon point-of-sale and hazard analysis for new development. The CGS is continually <br />working to expand the areas where their map is available and is currently mapping areas in <br />San Mateo and Contra Costa County; however, these maps are not expected to be <br />completed until 2016.14 <br />5. 3. Tsunamis & Seiches <br />Large underwater displacements from major underwater earthquake fault ruptures or <br />landslides can lead to ocean waves called “tsunamis.” Since tsunamis have high velocities, <br />the damage from a particular level of inundation is far greater than in a normal flood event. <br />Similarly, water sloshing in lakes during an earthquake, called “seiche,” is also capable of <br />producing damage. <br />Tsunamis can result from off-shore earthquakes within the Bay Area or from distant <br />events. It is most common for tsunamis to be generated by offshore subduction faults such <br />as those in Washington, Alaska, Japan, and South America. Tsunami waves generated at <br />those far-off sites can travel across the ocean and can reach the California coast with <br />several hours of warning time. Local tsunamis can also be generated from offshore strike- <br />slip faults. Because of their close proximity, we would have little warning time. However, <br />the Bay Area faults that pass through portions of the Pacific coastline or under portions of <br />the Bay are not likely to produce significant tsunamis because they move side to side, <br />rather than up and down, which is the displacement needed to create significant tsunamis. <br />They may have slight vertical displacements, or could cause small underwater landslides, <br />but overall there is a minimal risk of any significant tsunami occurring in the Bay Area from <br />a local fault. The greatest risk to the Bay Area is from tsunamis generated by earthquakes <br />elsewhere in the Pacific. <br />Though the Bay Area has experienced tsunamis, it has not experienced significant tsunami <br />damage. In 1859, a tsunami generated by an earthquake in Northern California generated <br />4.6 m wave heights near Half Moon Bay. The M6.8 1868 earthquake on the Hayward fault is <br />reported to have created a local tsunami in the San Francisco Bay. In 1960, California <br />experienced high water resulting from a magnitude 9.5 off the coast of Chile. The tsunami <br /> <br />13 Department of Conservation, Seismic Hazards Zonation Program Fact Sheet, California Geological Survey <br />14 Tim McCrink, CGS, Personal communication, April 3, 2015