ZCZC PWMWRKPNS DEF TTAA00 KPWM DDHHMM MEZ007>009-012>014-018>022-023>028-NHZ001>010-013-014-231200 PUBLIC INFORMATION STATEMENT NATIONAL WEATHER SERVICE GRAY ME 700 AM EST TUE MAR 22 2011 The National Weather Service and the National Oceanic and Atmospheric Administration have declared the week of March 20th through 26th Tsunami Awareness Week in Maine and New Hampshire, as well as in the remainder of the coastal United States. ...MECHANICS OF A TSUNAMI WAVE... Tsunami waves are generated by events that cause some initial displacement of water. This displacement can be caused by the movement of ocean floor, by underwater (or at the shoreline) landslides or land slumps, volcanic activity, or large releases of gasses from the ocean floor. Although very rare, a tsunami also can be initiated by a large meteorite or asteroid plunging into the ocean. Displacements of the ocean floor... Tsunami waves are often generated along fault lines in the earth's crust. Typically, major tsunamis are generated in areas where the continental and oceanic plates are in compression. As a general rule, the continental plate rides over top of the oceanic plate. While the edges of both plates are engaged (not moving with respect to each other), the compressive forces that result from the overall plate movement causes tension to build up between the plates in the overlapping area (called the subduction zone). Eventually, due to increasing pressure, the edges of the plates disengage, allowing the plates to shift. This fracturing and subsequent movement of the plates causes an earthquake and may produce a tsunami. Earthquakes can also generate tsunami by causing subterranean landslides and land slumps, especially near the edge of the continental shelf. Movement of a tsunami waves... Once generated, tsunami waves move can move rapidly across oceans. In open ocean, the speed and the height of the tsunami wave depends on the depth of the ocean floor. In areas of the Pacific where the ocean depth is 20,000 ft, tsunami waves are less than a foot high and move at speeds of about 550 mph - about the speed of a jet. However, the speed of a tsunami wave slows and the height increases as the wave encounters shallower water. In about 300 ft of water, a tsunami wave will slow to about 60 mph and in 30 ft of water the wave will slow to 20 mph. When tsunami waves reach the coast... As a tsunami wave approaches the coast, the speed of the wave slows and the height of the wave grows. Depending on which side of the subduction zone (or slide or slump zone) you are located, the initial movement of water along the coast may either recede out to the sea or increase toward and onto the land. The recession of water toward the sea may be a clue that a powerful tsunami wave may follow. In either case though, a deadly tsunami wave may be imminent. In addition, it is important to remember that the first wave may not be the largest. Tsunamis typically generate a series of waves and some of the later waves are often greater than the first. When a tsunami reaches shore, it may appear as a rapidly rising or falling tide, a series of breaking waves, or even a tidal bore. Reefs, bays, entrances to rivers, undersea features and the slope of the beach all help to modify the tsunami as it approaches the shore. While considerable attention is directed toward the flooding effects of the tsunami, much of the damage is due to the force generated by the rapid movement of water. Even in cases where no flooding of land occurs, the rapid movement of water in channels, bays, and harbors can cause considerable damage to boats, docks and marinas. In addition, the strong currents can become deadly and wash people out to sea. JENSENIUS NATIONAL WEATHER SERVICE GRAY, MAINE NNNN