As it continues towards land, should the trough of the tsunami reach a coastline first, the water level along the coastline appears to fall rapidly, as if the tide is ebbing. Figure 2c shows amplification of the local tsunami as it passes over the continental slope - this is due to the tsunami encountering shallower water. Because the distant tsunami is traveling through deeper water, it is moving much faster. 2b shows the initial tsunami split into a deep ocean (distant) and coastal (local) tsunami, headed in opposite directions. This abrupt motion displaces the overlying water upwards and downwards, initiating a tsunami.
In this situation, an earthquake results from a sudden shift in the subduction zone between continental and oceanic crusts (2a). Tsunamis can have periods upwards of an hour and wavelengths of several kilometers.įigure 2 shows the generation of a tsunami from an earthquake and how it travels across an ocean. The majority of wind-driven waves have periods of less than 20 seconds and wavelengths of several hundred meters. Wavelength (L) is the distance between subsequent crests (in this case, A and B), while period (T) is the time it takes for subsequent crests to pass the same point. The wave height (H) is the distance from the crest of the wave to its trough. Diagram of the characteristics of an ocean wave. However, when a tsunami reaches the continental shelf and begins to shoal, it will slow and increase in height.įigure 1. This is because its wavelength is on the order of several kilometers. In deep water, a tsunami can pass underneath a ship undetected. Thus, a tsunami travelling through water depths of 4,000 meters would be moving at approximately 200 m/s, or close to 450 miles per hour! At the same time, because of its very large wavelength, it is losing very little energy. The rate at which a tsunami loses its energy is inversely proportional to its wavelength. Where g is gravitational acceleration (9.81 m/s). Their velocity (C), like wind waves, is a function of water depth (h): However, unlike regular wind-driven waves, which generally have wavelengths of up to several hundred meters and periods less than 20 seconds, tsunamis can have wavelengths of several kilometers and periods anywhere from several minutes to upwards of an hour. The shape and characteristics of a tsunami are similar to wind-driven waves (Figure 1) - it has a wave crest, trough, wavelength (distance between two wave crests, A and B), and period (time it takes for crests A and B to pass a known point). The term was created by fishermen who returned from fishing and found everything devastated in the harbor though they didn't see or notice the wave in the open water (Wikipedia, 2004). The term tsunami originates from the Japanese words tsu (harbor) and nami (wave). Tsunamis are often mistakenly referred to as tidal waves, though tides play no role in their formation.
After such a disturbance, displaced water travels outward from its site of origin as a series of unusually large waves at great speeds (Komar, 1996). Background What is a tsunami?Ī tsunami is a wave produced by a disturbance that displaces a large mass of water - usually a result of geologic activities such as earthquakes, volcanic eruptions, underwater landslides, or in rare geologic cases, meteor strikes.
This Geologic Site of the Month provides background information and characteristics of tsunamis, some of their history in the Atlantic Ocean, and several possible locations where tsunamis could be triggered and impact the east coast of the United States in the future. However, although highly debated in the scientific realm, one of the world's "ticking time bombs" that may trigger a tsunami is located in the Atlantic Ocean! Although most people don't put "tsunami" and "Atlantic Ocean" in the same sentence, history and geology tell us that the Atlantic Ocean does experience tsunami activity, albeit on a less catastrophic scale. With the recent December 2004 tsunami that wreaked death and destruction in Indonesia, Sri Lanka, and beyond ( view an animation of the tsunami by the National Oceanic and Atmospheric Administration (NOAA) - Apple QuickTime required), many might wonder about tsunamis occurring in the Atlantic Ocean and striking the east coast of the United States most notably, Maine.