Tsunami Preparedness Week: What is a Tsunami?
In honor of Tsunami Awareness Week in Alaska, KTVA is giving you the top five things you need to know about tsunamis — starting with the definition.
A tsunami is one of the most destructible forces of nature. It is defined as a series of extremely long waves, sometimes hundreds of miles apart, caused by a large and sudden displacement of the ocean.
Tsunamis radiate out from the central point of origin and can travel across an entire ocean basin. As tsunamis reach the coast, they can cause a influx of water and strong currents that can last days.
A tsunami is caused by a large and sudden displacement of the ocean. This can happen in a few different ways.
Large earthquakes below or near the ocean floor are the most common cause of sudden displacement. In the tsunami database, 88 percent of the recorded tsunamis were caused directly or indirectly by earthquakes. Tsunamis can also be caused by landslides, volcanic activity, weather and even objects such as asteroids or comets.
The biggest threat in Alaska is earthquakes. An earthquake big enough and close enough to the earth's surface causes vertical movement in the ocean floor that can set a tsunami in motion. As the ocean floor raises or drops, the water near it does the same. The tsunami radiates out from the initial disturbance as the water of the ocean tries to regain balance. The type of earthquakes, size, proximity to the ocean floor, depth of water and duration of the earthquake all have effect on the size of a tsunami.
A tsunami is defined as a series of waves, often referred to as a wave train. In some areas, the set of waves can last for days.
The speed of a tsunami depends on the depth of the water. In deep water, speeds can reach that of a passenger jet in excess of 500 mph. In shallow water, they often travel at 20 to 30 mph.
The speed of travel can be calculated by taking the square root of the depth of the ocean multiplied by the acceleration due to gravity. This means that in the central pacific, with a water depth of 15,000 feet, a tsunami can travel at 475 mph.
In the open ocean, the distance between tsunami waves can often be hundreds of miles, while the height may only be a few feet. For this reason, many boats at sea don't even notice the passing waves.
As those deep-water waves approach land, the shallower water decreases the wavelength and increases the magnitude. The waves can be anywhere from barely noticeable to 100-feet high (in rare cases) when making landfall. A large tsunami can spread up to a mile inland.
Tsunamis rarely become a large, breaking wave when they reach the coast. Often times they look as if they are an increase in water height, similar to a fast-moving tide it moves inland. Sometimes, as tsunamis reach the shore, the water can recede before the wave itself makes landfall.
Tsunami Awareness Week
As Tsunami Awareness Week continues, we will look closer into ways you can prepare for the worst case scenario and let you know how you can get the latest information regarding these destructive waves.
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