Tsunamis are typically generated by underwater earthquakes—landslides, volcanic activity, and meteor strikes are other known, but less common, tsunami sources. Tsunami generating earthquakes usually occur in subduction zones, such as those found in the Pacific Ocean off the U.S. western and Alaskan coasts, as well as in the Caribbean. Marked by deep trenches in the seafloor, subduction zones are formed where one of the earth’s outer shell of tectonic plates plunges underneath another. Usually the plates are gradually moving past each other, but friction may temporarily lock them together, causing stress to build up between the plates. Sometimes the stress is relieved suddenly in the form of a large earthquake. As shown in figure 1, the bottom plate dives farther down, snapping the top plate violently upward, disturbing the overlying seawater. The size of the resulting tsunami depends on a complex set of factors, including the size of the earthquake, its depth below the ocean floor, the depth of the water, the type and amount of seafloor movement and the energy released. Once generated, some tsunami waves move quickly inland while other waves head toward the open ocean, often at speeds up to 600 miles per hour. Therefore, a tsunami generated by an earthquake off the coast of Alaska would be a local tsunami for that state’s coastal areas, and could strike within minutes of the event, while the same event is considered a distant tsunami for the coast of Washington state, which would not likely be hit until 3 or more hours later. While tsunamis can be a high impact natural hazard, the frequency of damaging tsunamis in the United States has been low, compared with other natural hazards. According to NOAA’s records, the last tsunami causing significant impacts was at Skagway, Alaska, in November 1994, where the landslide and associated tsunami caused one death and $25 million in damages. According to FEMA, flooding, severe storms, and hurricanes are the most common and costly causes of disaster declarations in the United States; at least 10 such events since 1989 have each required FEMA relief expenditures in excess of a billion dollars. Although damaging tsunamis are relatively rare, the devastation caused by the Indian Ocean tsunami demonstrates the need for assessing the threat, and for monitoring and preparing for an event in at-risk areas, particularly low-lying, seismically active coastal areas. The West Coast/Alaska Tsunami Warning Center in Palmer, Alaska, is responsible for warning Alaska, the west coast and east coast states, and states along the Gulf of Mexico, while the Richard H. Hagemeyer, Pacific Tsunami Warning Center in Ewa Beach, Hawaii, is responsible for warning Hawaii and U.S. territories in the Pacific Ocean and Caribbean Sea. These warning centers use two types of data for determining when to issue a tsunami warning. First, they receive and analyze earthquake data from seismic networks operated by NOAA, USGS, the states, and universities to determine whether to issue a warning. If the seismic data indicate that a local tsunami may be generated, the responsible warning center issues a warning based on the earthquake data alone. Second, the warning centers analyze sea-level data to determine whether a tsunami has actually been generated, and if not, cancel the warning. The centers receive sea-level data through a network of DART stations and sea-level gauges, as shown in figure 2. DART stations consist of a seafloor bottom-pressure recording system that is capable of detecting tsunamis smaller than 1 inch and is connected to a surface buoy that transmits the data by satellite to NOAA. Scientists at the warning centers incorporate the data from the DART stations into tsunami forecast models to estimate the size of the expected waves and the potential impact on coastal areas. The tsunami warning centers have used forecast models they developed, as well as models developed by NOAA’s Pacific Marine Environmental Laboratory, which produce expected tsunami inundations at nine high-risk locations. A network of federal, state, and local government agencies are responsible for ensuring that a tsunami warning reaches the public. Figure 3 provides an overview of the key components of this process. The federal tsunami warning centers send a warning to NWS forecast offices and state emergency management centers by multiple means, such as FEMA’s National Warning System, a dedicated telephone hotline, and NWS’s satellite-based National Weather Wire Service. The forecast offices, in turn, transmit the warning over NOAA Weather Radio and the Emergency Alert System (EAS). State emergency managers receive tsunami warnings from NWS and then warn counties and local communities using multiple methods, including a dedicated telephone network for state and local emergency management officials. Finally, county and local officials are responsible for warning the public and issuing evacuation orders, using a variety of methods including bullhorns, sirens, and telephone systems. Federal, state, and local government agencies also conduct hazard mitigation activities to reduce the potential impacts of tsunamis. At the federal level, NWS operates the TsunamiReady community recognition program. Initiated in 2000, TsunamiReady is modeled after NWS’s StormReady program for hurricanes and tornados. NWS meteorologists in regional forecast offices are responsible for reviewing applications from coastal communities and ensuring that they meet program requirements in conjunction with state emergency management officials. NOAA also provides a chairperson and funding for the NTHMP. From 1998 through 2001, NOAA provided $2.3 million annually for the NTHMP, increasing to $4.3 million annually in 2002 through 2005, and returning to $2.3 million in 2006. Initially, the five participating states each received less than $100,000 annually from the NTHMP, but in recent years they have each received approximately $275,000 annually to directly supplement their individual mitigation efforts, while NOAA and the other federal partners used the remaining funds to support their own activities under the program. NOAA estimates that since the program’s inception the states have matched the NTHMP funding by a ratio of six state in-kind or dollar contributions for every program dollar. Tsunamis pose the greatest hazard to the coastal areas of the five states bordering the Pacific Ocean and U.S. territories in the Caribbean, but for many of these areas reliable, comprehensive assessments of potential tsunami impacts on people and infrastructure have not been completed. Some high-hazard coastal areas do not have tsunami inundation maps— the foundation for evaluating potential tsunami impacts on communities— showing the extent to which a tsunami would penetrate inland and flood communities, while others have maps that may not be reliable. Progress in developing these maps has been slow, primarily because accurate maps are complex and costly for states to produce. Furthermore, states and communities do not have comprehensive information on the potential human, structural, and economic impacts of a tsunami. While FEMA has standardized computer software for estimating losses resulting from natural disasters such as floods, hurricanes, and earthquakes, no such tool exists for assessing tsunamis. According to NOAA, the general areas most threatened by both distant and local tsunami hazards are Hawaii and the west coast states of California, Oregon, and Washington, whereas Alaska and the Caribbean Islands of Puerto Rico and the U.S. Virgin Islands are threatened primarily by local tsunamis, as shown in table 1. The hazard levels are primarily based on tsunami source, height, and frequency information since 1900—the most reliable and accurate information available—from NOAA’s National Geophysical Data Center tsunami database. According to NOAA, Hawaii is a high-hazard area for distant and local tsunamis. Hawaii has experienced many destructive tsunamis because of its location in the Pacific Ocean, as shown in figure 4, where about 80 percent of all recorded tsunamis have occurred. More than one-half of all tsunamis recorded in the Hawaiian Islands were generated in the distant Aleutian regions of the northern and northwestern Pacific Ocean, and about one-fourth were generated along the western coast of South America. Hawaii’s local tsunami threat stems from earthquake and volcanic activity, which cause underwater landslides off the coast. Hawaii suffered its greatest tsunami death and destruction in 1946, when an earthquake in the Aleutian Islands generated a tsunami that reportedly killed 159 people. Hilo, Hawaii suffered the greatest loss—96 deaths and the destruction of its waterfront area. Since 1946, an additional five tsunamis—four distant and one local—have caused a reported 63 deaths and widespread destruction. NOAA considers the west coast a high-hazard area for distant tsunamis and medium-hazard area for local tsunamis. Like Hawaii, the west coast historically has suffered the most destruction from tsunamis generated by Pacific earthquakes in the distant South America and Aleutian regions. In California, two tsunamis have caused significant damage. The 1960 Chilean earthquake caused estimated tsunami damages of over $1 million, and the tsunami generated by the 1964 Alaskan event killed 12 in Northern California and caused an estimated $15 million in destruction, including damages inside San Francisco Bay. Oregon and Washington both have sustained damages in coastal areas from distant tsunamis over the years. Although distant tsunamis historically have been most common, a local tsunami generated by the 750 mile long Cascadia subduction zone, lying just 50 to 100 miles off the coasts of Washington, Oregon, and Northern California, is considered a major threat. (See fig. 5.) Geologic and other records from a Cascadia earthquake in 1700 suggest that the fault could generate a tsunami wave of up to 30 feet that would likely reach the Oregon coast in 15 to 30 minutes, raising concerns of a catastrophic future event.