Tsunamis that threaten the coast of Maine can originate in the open ocean or within the Gulf of Maine. Those originating outside the Gulf of Maine can produce damaging impacts along the entire Maine coast while those originating in the Gulf of Maine are likely to have more localized impacts.
The shallow waters of Georges Bank and the continental shelf greatly lessen the threat of tsunamis in Maine. When tsunami waves that are generated outside the Gulf of Maine encounter the shallow waters of Georges Bank, the tsunami loses speed and begins to break and a portion of the wave energy is reflected back out to sea. Wave energy, however, can still permeate the Gulf of Maine through the deeper water of Northeast and Great South Channels. The deeper water allows the waves to continue to travel at faster speeds through the channels than in the shallower waters on either side. Much of the incoming tsunami wave energy would then be refracted to the right and left of the channels and that initial energy would be redirected toward Georges Bank and Nova Scotia. However, ensuing reflections and refractions of this wave energy would lead to rapid and chaotic fluctuations of water levels in the Gulf of Maine that could last for more than 6 hours. These fluctuations could cause strong, dangerous, and damaging currents along the coast and could cause inundation at the time of high tide.
The two most significant tsunami sources outside the Gulf of Maine would be a major earthquake along the subduction zone of the Puerto Rican Trench (just to the north of Puerto Rico) or a large sediment slide on the continental slope of the East Coast. If a tsunami were to originate in the Puerto Rican Trench, much of the tsunami wave energy would be directed north toward the Gulf of Maine. This tsunami would reach Maine between 5 and 6 hours after the earthquake, which allows time for a warning or advisory. Although this tsunami poses a threat to Maine, the impacts to northern New England of a tsunami generated along the Puerto Rican Trench would be relatively small compared to those on Puerto Rico and Bermuda.
A large sediment slide or slump along the continental slope can also generate a tsunami with the potential to enter the Gulf of Maine. The exact impacts of slide or slump depend on the orientation of the slide or slump with respect to the Northeast and Great South Channels and magnitude of the slide or slump.
Sediment slides or slumps in the Gulf of Maine can also cause tsunami waves, though these are more likely to be localized. Slides or slumps are most likely to occur near the coast or along the northern edge of one of the banks or ledges in the Gulf of Maine. The exact impacts of the tsunami waves depend on the orientation and magnitude of the movement of the continental slope. There would be little warning time for the subsequent tsunami because of their proximity to the coastline of Maine.
Locally sourced tsunamis can also be generated by the atmospheric pressure waves. Also known as meteotsunamis, these waves are caused by a harmonic resonance between the atmosphere and the ocean. This resonance allows the ocean wave to grow with time as both waves move in unison. Since the forward speed of the wave is determined by the depth of the ocean, the atmospheric feature needs to move at the just the right speed as determined by the ocean depth. In the Gulf of Maine, very fast moving weather systems can meet this requirement. The movement of both the atmospheric and the ocean waves need to move toward the coast to threaten Maine. A meteotsunami caused the unusual tidal fluctuations that occurred along the mid-coast in October 2008.
Mechanics of a Tsunami Wave
Tsunami waves are generated by events that displace of water. This displacement can be caused by movement of the ocean floor, by underwater (or at the shoreline) landslides or land slumps, volcanic activity, large releases of gases from the ocean floor, atmospheric pressure waves, 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 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 subduction zone. Eventually the increasing pressure causes the edges of the plates disengage, allowing the plates to shift. The earthquake that results from the fracturing and subsequent movement of the plates may produce a tsunami. Earthquakes can also generate a tsunami by causing subterranean landslides and land slumps, especially near the edge of the continental shelf.
Movement of tsunami waves...
Tsunami waves move rapidly across oceans one generated. The speed and 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 feet, tsunami waves are less than a foot high and move at speeds of about 550 mph - about the speed of a jet. The tsunami wave length can stretch nearly 100 miles and this large wavelength allows the tsunami to travel great distances while losing little energy. As the wave encounters shallower water the speed of a tsunami wave slows and the height increases. In about 300 feet of water, a tsunami wave will slow to about 60 mph and in 30 feet of water the wave will slow to 20 mph.
When tsunami waves reach the coast...
As a tsunami wave approaches the shallower depths near the coast, the speed of the wave slows and the height of the wave increases. 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 is strong indication that a powerful wave is approaching. If the tide is behaving in a way that it is not supposed to, or is otherwise unpredictable, a deadly tsunami wave may be imminent. If this is the case, leave the beach. It is also important to remember that tsunamis typically generate a series of waves and the first wave may not be the largest
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.
Georges Bank shelters much of the Maine coast from a major tsunami threat. However, small but dangerous tsunami waves are possible. Strong currents generated by rapid and chaotic changes of water levels in the Gulf of Maine present the greatest threat to Maine. Inundation of land areas is possible at the time of high tide. However, this threat is not as severe in Maine as the threat of the fluctuation of water changes.
Staying Safe When a Tsunami Threatens
Watches, Warnings, and Advisories
The National Tsunami Warning Center (NTWC) in Palmer, Alaska, issues tsunami watches, warnings, and advisories for the East Coast. The Center monitors seismic activity throughout the world and determines the likelihood of tsunamis based on that activity.
A tsunami watch is issued based on seismic information when a tsunami is possible for a particular location. The watch provides advance notification to areas that possibly could be impacted by a destructive tsunami, and the recommended action is to stay alert for more information. As more information becomes available, the watch may be changed to a warning or advisory, or cancelled. During a watch, the Tsunami Warning Center will issue updated information at least every hour.
A tsunami advisory is issued when a tsunami with the potential to generate strong currents or waves dangerous to those in or very near the water is imminent, expected, or occurring. Inundation of land areas is not expected in the case of an advisory, though the advisory may be upgrade, downgraded, extended, or cancelled as appropriate. Recommended actions for those in an advisory area are get off beaches and to evacuate harbors and marinas if there is time to safely do so.
A tsunami warning is issued when a tsunami with the potential to generate widespread inundation is imminent, expected, or occurring. In the case of a tsunami warning, it is recommended that local officials evacuate low lying areas and reposition ships to deeper waters when there is time to do so. Warnings may be updated, adjusted geographically, downgraded, or canceled.
The National Tsunami Warning Center issues information statements to keep the public informed of seismic situations when a watch, warning, or advisory has been issued by the NTWS for another section of the ocean.
During a tsunami event affecting the Gulf of Maine, the National Weather Services offices in Gray and Caribou will issue special weather statements that provide updated information on the event and its impact along the northern New England coast.
Staying Safe in the Gulf of Maine
Strong tsunami-generated currents along the coast and in the channels and bays of Maine present the greatest threat to those in the Gulf of Maine. Tsunamis generated in the Gulf of Maine are not likely to provide significant warning time. If water levels unexpectedly start to recede or increase or if strong currents unexpectedly develop, a tsunami has arrived. It is imperative to get off beaches and move away from the water's edge to higher ground immediately once this behavior of the tide is observed.
There will likely be warning time if a distant event such as an earthquake in the Puerto Rican Trench is forecast to produce a tsunami in the Gulf of Maine. Once the tsunami waves reach the Gulf of Maine, keep distant from the water's edge. It is important to remember that during a tsunami event in the Gulf of Maine, water levels will fluctuate rapidly and chaotically and surge size can vary extensively long local areas in Maine. Do not approach the water's edge until emergency officials or the National Weather Service indicate that the dangerous conditions have subsided.