Properties
| As the water is propelled away from the center of the underwater disturbance, this water level fluctuation travels through the ocean at an average rate of approximately 400 miles per hour. Tsunami waves can cross the Pacific Ocean in about a day. The change in the ocean's water level ripples out from the disturbance epicenter in successive masses of water, slowing as it reaches more shallow areas. Much as the land is folded into mountains and valleys by an earthquake, water folds into temporary mountains and valleys. These mountains or valleys of water travel over a depth of 18,000 feet water at a rate of 520 mph, slowing to 212 mph over a depth of 3000 feet, and finally slowing to 30 mph at a depth of 60 feet. As it slows, the water displacement becomes more dramatic and the wave heightens. The tsunami wave that may have been less than 10 feet in deep water can grow to 50 feet by the time it reaches shore. This heightening, called 'shoaling,' occurs in all waves in a shallow area. Because tsunamis usually displace a large volume of water, the wave may not crest as a normal wave does. If an underwater valley was created at the seismic epicenter there is most often a 'drawdown' of water away from shorelines, exposing underwater landscape. If an underwater mountain was created by volcano, landslide, meteorite, or earthquake, there will be a dramatic increase in water level, creating a tsunami that washes inland. The tsunami wave usually 'runs up' only about 15 feet above sea level. 'Run-up' measures how high the tsunami runs up above sea level. The greatest run-up is caused by volcanic eruptions or by seismic events close to the coast. Flooding lasts longer in more landlocked areas, with open areas allowing water to flow back to a sea, the water level of which can take days to normalize. Tsunami waves have longer periods (intervals between waves) than and up to 10 times the velocity of wind waves. Gravity plays an important role in tsunamis, however, as displaced water seeks to level out and cling to the earth in a uniform manner. Gravity pulls the displaced mountain of water toward the earth's center, flattening the mountain of water as it travels. Although tsunamis have been called "tidal waves" they are not; they have no relation to the tides. Unlike an individual wave fighting the gravitational pull of the moon, a tsunami is an ocean-wide occurrence primarily influenced by the earth's gravity. A tsunami wave crests only if the topography of a shoreline causes it to do so; lunar gravity and wind has little noticeable effect on a tsunami wave. After water is displaced by earth and a tsunami is generated, the ocean tries to regain its equilibrium in relation to the earth's center, and thus the tsunami lowers over distance, though the wave loses little energy. The rate at which a tsunami wave dissipates is hard to immediately gauge, as it depends on the force with which the water was initially displaced. Generally, tsunamis have a 50-mile wavelength and a period between crests that lasts about an hour. This long wavelength shortens the ratio between wavelength and ocean depth. Because tsunami waves have a low ratio between wavelength and depth, they behave in deep water as waves do in shallow water: they travel long distances with little energy loss. Because tsunami waves are generated by pressure, pressure sensors anchored to the ocean floor can transmit warning signals to buoys floating on the water's surface to indicate a pressure increase. The increase in water pressure indicates tsunami-like conditions. Extended periods between waves is another indicator. If the third major indicator, seismic activity, is present, the tsunami center receiving the signals from the buoy via satellite will usually issue a tsunami warning. If the first motion detected is a valley of water ('down-trough') then scientists know that the plates subducted downward, creating a valley on the ocean floor. Likewise, if the first motion detected is a mountain of water ('up-crest'), then scientists know that shifting plates created a mountain on the ocean floor. It is important to note that the ocean's behavior following seismic activity is confusing. Earthquakes and consequential landslides can cause successive tsunamis, the first wave of which is not necessarily the greatest. Each tsunami has its own succession of dissipating waves. The water level from the first tsunami may abate to normal levels, only to dangerously rise again as tremors, aftershocks, landslides, or volcanoes in different locations have caused new tsunamis in the hours and days after the initial tsunami was generated. Scientists can clearly discern the distinct wave patterns while observers on the ground may not. |