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the Everett area usually occur in unconsolidated or partially <br /> consolidated sediments. When these soils are combined with steep <br /> slopes the soils are left in an unstable condition. When these <br /> unstable formations become saturated with water, they become more <br /> likely to succumb to the forces of gravity. When unconsolidated <br /> soils are underlain by or are interbedded with a highly <br /> impermeable soil formation such as compacted glacial till, they <br /> become saturated during heavy rains, because the water cannot <br /> rapidly seep into the underlying non-porous material. <br /> Unconsolidated soils, steep slopes, saturation of permeable <br /> soils above or beneath impermeable formations combine with <br /> gravitational forces to cause landslides. Human induced factors <br /> can also increase the likelihood of landslides. These actions <br /> include diversion of water from rooftops and paved areas, <br /> improperly placed and compacted fills, dumping of debris, road <br /> and utility cuts into hillsides, excavation for building sites, <br /> and failure of retaining walls. When such human activities are <br /> combined with the other factors mentioned, the potential for <br /> landslides increases. <br /> Seismic Hazards <br /> Seismic hazards in the Everett and Puget Sound area consist of <br /> two kinds, ground shaking and ground failure. Rupture along <br /> fault lines is not a factor since the faults are located <br /> thousands of feet below glacial deposits and they are not <br /> discernible at the surface. The consequences of seismic events <br /> are two types of hazards: <br /> 1. landslides <br /> 2. liquefaction <br /> Landslides which are likely to occur as a result of a seismic <br /> event are the same areas and formations where landslides are <br /> likely to occur under other non-seismic conditions. Seismic <br /> activity may trigger landslides in areas of landslide hazard. <br /> Liquefaction is a phenomenon where soil loses strength and its <br /> bearing capacity during an earthquake. This phenomenon is most <br /> likely to occur on non-cohesive soils common to post-glacial <br /> deposits such as alluvium (as in river floodplains) or in areas <br /> which have been filled by human activities, particularly when <br /> these soils have a high moisture content. Such soils are poorly <br /> compacted and when moist conditions are present, an earthquake <br /> will cause the soil to liquefy. Certain engineering and <br /> construction methods may reduce the likelihood of damage to <br /> structures resulting from an earthquake. The Uniform Building <br /> Code regulates the design and construction of buildings located <br /> in seismic hazards areas. No additional zoning requirements are <br /> necessary to regulate structural design. Identification of such <br /> areas is necessary in order to evaluate development proposals in <br /> areas which may be prone to liquefaction. Coastal areas which <br /> have been filled, such as those at the Port of Everett, are <br /> candidates for liquefaction during an earthquake. Upland areas <br /> where liquefaction may occur are sites which have been filled <br /> without proper engineering and compaction, sites affected by <br /> river deposited alluvium, and wetland areas. Where the layer of <br /> 3 <br />