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D.6 Tsunami Tsunamis are fast-moving waves triggered by earthquakes or other events that displace large volumes of water, such as underwater landslides or volcanic eruptions. When these waves reach the shore, they can build to great heights and inundate large areas of dry land. Situation and Assumptions Tsunamis may be triggered by earthquakes or other events occurring hundreds or thousands of miles away. Such events would have a smaller impact on Redwood City than on communities near their place of origin but could still create emergency conditions in Redwood City. Redwood City would likely have little warning (an estimated 10 to 15 minutes, at most) of tsunamis from local events. Strong local earthquakes may not necessarily generate tsunamis, or such tsunamis may be inconsequential. Tsunamis from distant events would likely take at least an hour, and in some cases several hours, to reach Redwood City. A similar phenomenon called a seiche may occur in enclosed or partially enclosed bodies of water, such as San Francisco Bay. A seiche is a standing wave that "sloshes" back and forth across the body of water. Although usually small, seiches triggered by earthquakes may be significant and have similar effects as a major tsunami. Redwood City's zone at risk of tsunami inundation is mostly limited to bayshore islands and sloughs. Areas along the shoreline, including the Port of Redwood City, and along the banks of Redwood Creek, may be at risk. The risk zone includes the US 101 bridge over Redwood Creek. A tsunami or seiche event generally involves multiple waves. The first wave is often not the largest. A tsunami or seiche event may cause one or more of the following direct impacts in Redwood City: • Damage or destruction to moored boats, as well as docks, marinas, ports, and related infrastructure. • Water damage to vehicles and buildings • Structural damage to homes and nonresidential buildings along shorefronts • Road and street closures or congestion from flooding or debris • Destruction of power lines and power outages • Communication system outages • Road and street congestion from traffic light outages • Disruptions to water and wastewater services • Releases of hazardous materials • Fires from natural gas leaks or damaged electrical equipment • In severe instances, significant injuries or death to community members D-73