Figure 5 Regional Fault Lines

FIGURE 5 REGIONAL FAULT LINES

Other Seismic Hazards In addition to the direct physical damage that can result from the motion of the earthquake, damage can result from liquefaction. Liquefaction occurs where water-logged soils near the ground surface lose compaction during strong ground motion. This can cause building foundations to shift and can result in significant structural damage. Soils susceptible to liquefaction are typically found in areas of low-lying, current, or former floodplains. Portions of the city within or directly adjacent to the floodplains of the San Pablo Creek are areas of the greatest peak ground acceleration. These areas are likely to experience the strongest shaking during an earthquake event, which could result in severe liquefaction. However, a majority of the city is within an area of low or very-low liquefaction risk. Very high liquefaction risk is limited to the areas adjacent to San Pablo Creek and Lauterwasser Creek. Figure 6 shows the areas facing an elevated liquefaction risk in Orinda.

While most of the city is not susceptible to liquefaction, it is a potentially destructive secondary effect of strong seismic shaking. Liquefaction is the complete failure of soils, occurring when soils lose shear strength and flow horizontally. Liquefaction occurs primarily in saturated, loose, fine- to mediumgrained soils in areas where the groundwater table is within approximately 50 feet of the surface. Shaking causes the soils to lose strength and behave as liquid. Excess water pressure is vented upward through fissures and soil cracks and can result in a water-soil slurry flowing onto the ground surface. This subsurface process can lead to near-surface or surface ground failure that can result in property damage and structural failure. Groundwater that is less than 10 feet to the surface can cause the highest liquefaction susceptibility, with lower groundwater levels causing lower liquefaction risks. Site-specific geotechnical studies are the only practical and reliable way of determining the specific liquefaction potential of a site; however, a determination of general risk potential can be provided based on soil type and depth of groundwater.

In most cases, proper design and construction of subgrade soils and building foundations provides a mechanism to mitigate the risk of seismic hazards to an acceptable level in conformance with the California Building Code. The representation of areas having a liquefaction potential is only intended as notification to seek further site-specific information and analysis of this potential hazard as part of future site development. It should not be solely relied upon, without site-specific information and analysis, for design or decision-making purposes.

Seismic seiches are waves that can occur in a body of water as a result of seismic shaking. Orinda has a large wastewater treatment facility located within its city limits that could be damaged and/or shutdown during a seiche event within the tanks, although this is considered unlikely.