The awesome power of tsunamis can have deadly consequences. In the past century, tsunamis have killed more than 50, people. In and alone, major tsunamis hit Nicaragua, Indonesia, and Japan—with a combined death toll of 2, On July 17, , an earthquake off the coast of Papua New Guinea triggered three devastating tsunamis, each about 30 to 40 feet 9 to 12 meters high. The massive waves annihilated several of the island's villages and killed about 2, people; thousands more were injured.
What is a Tsunami? Tsunamis are popularly known as "tidal waves," even though they have nothing to do with tides. Somewhat more accurate is the translation of the Japanese term tsunami: "harbor wave. Indeed, the islands of Japan have been hit by more tsunamis than any other country.
Many scientists use the term seismic wave to describe tsunamis, since these waves are often created by seismic, or earthquake, activity along the ocean floor. Tsunamis can also arise through a sudden and massive undersea movement, such as a landslide or volcanic eruption. Tsunamis can occur in any ocean. However, they are most likely to arise in the Pacific Ocean's so-called "Ring of Fire.
Anatomy of a Killer Be it an undersea earthquake, landslide, or volcanic eruption, the event that triggers a tsunami typically lasts a few seconds or, at most, a minute or two. Yet the tsunamis that result can continue to pound nearby shores for hours, and distant shores for days. Indeed, a tsunami is rarely a one-hit phenomenon. Just as a single rock thrown into a lake creates many ripples spreading out from the impact, the force that spawns tsunamis sends out a series of underwater shock waves.
But this force is not a single point, like the rock. The fault line on which an earthquake is centered can extend for hundreds of miles. In a tsunami-forming earthquake, the ground on one side of a fault line suddenly lifts or sinks.
All of the water above it also rises or falls, forming a high point called a wave crest or low point a wave trough in the water. In this way, each shudder or jolt of the earthquake sends out another tsunami front. In the very deep water of the open ocean, tsunamis travel quickly and spread out from one another. The spacing within a series of seismic waves can be as much as miles kilometers.
A newborn tsunami traveling in open water is virtually invisible. The surface of the ocean generally rises and falls only a few inches—or at most a few feet—between the wave's crest and trough.
As a consequence, tsunamis cannot be reliably spotted by airplane. Even observers in a ship would hardly feel the wave's passage beneath them because the bulk of a tsunami lies beneath the surface. The rise and fall of its wave reaches all the way down to the seafloor. Importantly, tsunamis do not die out after traveling a short distance as a windblown surface wave might. If the tsunami is caused by a local landslide, both its initial wavelength and period will be shorter.
The period of the tsunami waves may range from 5 to 90 minutes. The wave crests of a tsunami can range from a few to a hundred kilometers or more apart as they travel across the ocean. As the waves approach the coast, their wavelength decreases and wave height increases. On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers.
Key resources include the following:. A tsunami is caused by a large and sudden displacement of the ocean. Large earthquakes below or near the ocean floor are the most common cause, but landslides, volcanic activity, certain types of weather, and near earth objects e.
Earthquakes provide the energy to generate tsunamis through sudden movements to the water column. Key earthquake characteristics that contribute to tsunami generation are location, magnitude, and depth. Most tsunamis are generated by earthquakes with magnitudes over 7.
Generally, an earthquake must exceed magnitude 8. An earthquake must be big enough and close enough to the ocean floor to cause the vertical movement of the ocean floor that typically sets a tsunami in motion. As the ocean floor rises or drops, so too does the water above it.
As the water moves up and down, seeking to regain its balance, the tsunami radiates out in all directions. The amount of movement of the ocean floor, the size of the area over which it occurs which may be reflected in how long the earthquake lasts , and the depth of the water at its source are all important factors in the size of a resulting tsunami.
Earthquakes can also cause landslides that generate tsunamis. Source: Global Historical Tsunami Database. To learn more about earthquakes, visit the U. Most of the earthquakes that generate tsunamis occur on thrust or reverse faults. These earthquakes originate mainly where tectonic plates move toward each other in subduction zones. But, percent of damaging tsunamis are generated by strike-slip earthquakes, where the movement of the earth is horizontal.
These tsunamis are likely generated by associated landslides, movement of a sloping ocean floor, or the presence of seamounts, which are underwater mountains that can act like paddles and push the water horizontally. Tsunamis generated by strike-slip earthquakes normally affect regions near the source only.
To learn more about faults, visit the visual glossary from the U. Geology Survey. The largest earthquake ever recorded was a magnitude 9. This earthquake and the second largest earthquake, the magnitude 9.
Learn more about past earthquakes from the U. Geological Survey. Tsunamis can be generated when a landslide enters the water and displaces it from above subaerial or when water is displaced ahead of and behind an underwater submarine landslide. Tsunami generation depends on the amount of landslide material that displaces the water, the speed it is moving, and the depth it moves to.
Landslide-generated tsunamis may be larger than seismic tsunamis near their source and can impact coastlines within minutes with little to no warning, but they usually lose energy quickly and rarely affect distant coastlines. Most landslides that generate tsunamis are caused by earthquakes, but other forces like gravity, wind, and increased precipitation can cause overly steep and otherwise unstable slopes to suddenly fail.
Earthquakes that are not large enough to directly generate a tsunami may be large enough to cause a landslide that in turn can generate a tsunami. A landslide-generated tsunami may occur independently or along with a tsunami directly generated by an earthquake, which can complicate the warning process and compound the losses.
Examples of landslide-generated tsunamis: July 17, Papua New Guinea —A moderately sized magnitude 7. Three waves, the highest measuring roughly 49 feet high, struck the coast within 20 minutes of the earthquake, destroying entire villages. Approximately 2, lives were lost, and more than 10, people were displaced. July 10, Southeast Alaska —A magnitude 7.
A rock fall into Lituya Bay sent water surging over the opposite shore, clearing trees around the bay up to a maximum height of 1, feet. It is considered the largest tsunami ever recorded. To learn more about landslides, visit the U. Tsunamis generated by volcanoes, both above and below water, are infrequent, but several types of volcanic activity can displace enough water to generate destructive tsunamis. These include:. Like other nonseismic tsunamis, such as those generated by landslides, volcanic tsunamis usually lose energy quickly and rarely affect distant coastlines.
To learn more about volcanoes, visit the U. Air pressure disturbances often associated with fast moving weather systems, like squall lines, can generate tsunamis.
Meteotsunamis are regional, and certain parts of the world are prone to them due to a combination of factors such as local weather patterns and the shape and features of the surface of the Earth, both above and below the ocean.
Examples of meteotsunamis: June 13, Northeastern United States animation —Tsunami-like waves crashed upon the New Jersey and southern Massachusetts coasts, despite clear skies and calm weather. In Barnegat Inlet, New Jersey, three people were injured when a six-foot wave swept them off a jetty and into the water. After ruling out other sources, scientists determined the waves had been generated by a derecho a high-speed windstorm associated with a strong band of thunderstorms that had passed through the area hours earlier.
June 21, Vela Luka, Croatia —Without warning and during relatively nice weather, flooding waves inundated the port town of Vela Luka. Scientists ultimately identified the source as atmospheric and deemed it the strongest meteotsunami on record. This event featured It is very rare for a near earth object like an asteroid or comet to reach the earth, and there is still a lot of uncertainty about their potential to generate tsunamis and the size and reach of those tsunamis if they do.
Scientists believe there are two ways near earth objects could generate a tsunami. Large objects approximately 1, meters, 0. A tsunami is a series of waves, not just one.
These waves are often referred to as the tsunami wave train. A large tsunami may continue for days in some locations. The speed of a tsunami depends on the depth of the water it is traveling through. The deeper the water; the faster the tsunami. In the deep ocean, tsunamis can move as fast as a jet plane, over mph, and can cross entire oceans in less than a day. As the waves enter shallow water near land, they slow to the speed of a car, approximately 20 or 30 mph.
Tsunami speed can be computed by taking the square root of the product of the water depth and the acceleration of gravity In 15, feet of water, this works out to about miles per hour. At rates like this, a tsunami will travel from the Aleutian Islands to Hawaii in about five hours; or from the Portugal coast to North Carolina in eight and a half hours. In the deep ocean, the wavelength of a tsunami the distance between waves may be hundreds of miles, but its waves may be barely noticeable and are rarely more than three feet high.
Mariners at sea will not normally notice tsunamis as they pass beneath their hulls. As the waves enter shallow water near land and slow down, their wavelengths decrease, they grow in height, and currents intensify. When they strike land, most tsunamis are less than 10 feet high, but in extreme cases, can exceed feet when they strike near their source.
The first wave may not be the last or the largest. A large tsunami can flood low-lying coastal areas more than a mile inland. Not all tsunamis act the same, and an individual tsunami may affect coasts differently due to offshore and coastal features. Reefs, bays, entrances to rivers, undersea features, and the slope of the beach can all influence the size, appearance, and impact of tsunamis when they strike the coast.
A small nondestructive tsunami in one place may be very large and violent a few miles away. When a tsunami reaches the coast, it may look like a fast-rising flood, or a wall of water bore. Its appearance may differ at different points along a coast. It will not look like a normal wind wave. Knowledge of the history of tsunamis in your area is a good indicator of what is likely to happen in a future tsunami event. They may be more like a very rapidly rising tide.
This may be accompanied by much underwater turbulence, sucking people under and tossing heavy objects around. Entire beaches have been stripped away by tsunamis. News reports so far suggest that more than , people may have lost their lives, many of them washed out to sea. The most damaging tsunami on record before was the one that killed an estimated 40, people in following an earthquake in the South China Sea.
In some 36, people were killed by tsunamis in the South Java Sea, following the eruption of Indonesia's Krakatoa volcano. In northern Chile more than 25, people were killed by a tsunami in But tsunamis have been generated in other bodies of water, including the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans. North Atlantic tsunamis included the tsunami associated with the Lisbon earthquake that killed as many as 60, people in Portugal, Spain, and North Africa.
This quake caused a tsunami as high as 23 feet 7 meters in the Caribbean. Some were generated locally and others were the result of events far away, such as the earthquake near Portugal. The combined death toll from these Caribbean tsunamis is about 9, If you feel a strong quake do not stay in a place where you are exposed to a tsunami.
If you hear of an earthquake be aware of the possibility of a tsunami and listen to the radio or television for additional information. Remember that an earthquake can trigger killer waves thousands of miles across the ocean many hours after the event generated a tsunami. If you see the ocean receding unusually rapidly or far it's a good sign that a big wave is on its way. Go to high ground immediately. Many people were killed by the Indian Ocean tsunami because they went down to the beach to view the retreating ocean exposing the seafloor.
Apparently they were unaware that this phenomenon precedes a killer wave. Experts believe that a receding ocean may give people as much as five minutes' warning to evacuate the area.
An approaching tsunami is not something to be admired unless you are safely on high ground. The danger from a tsunami can last for several hours after the arrival of the first wave. A tsunami wave train may come as a series of surges that are five minutes to an hour apart. The cycle may be marked by a repeated retreat and advance of the ocean.
Stay out of danger until you hear it is safe.
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