Tsunamis

Tsunami as Waves

Tsunami are deadly ocean waves from the sharp jolt of an undersea earthquake. Less frequently, these waves can be generated by other shocks to the sea, like a meteorite impact. Fortunately, few undersea earthquakes, and even fewer meteorite impacts, generate tsunami.

Wave Height

Tsunami waves have small wave heights relative to their long wavelengths, so they are usually unnoticed at sea. When traveling up a slope onto a shoreline, the wave is pushed upward. As with wind waves, the speed of the bottom of the wave is slowed by friction. This causes the wavelength to decrease and the wave to become unstable. These factors can create an enormous and deadly wave.

Landslides, meteorite impacts, or any other jolt to ocean water may form a tsunami. Tsunami can travel at speeds of 800 kilometers per hour (500 miles per hour).

Wavelength

Since tsunami are long-wavelength waves, a long time can pass between crests or troughs. Any part of the wave can make landfall first.

In 1755 in Lisbon, Portugal, a tsunami trough hit land first. A large offshore earthquake did a great deal of damage on land. People rushed out to the open space of the shore. Once there, they discovered that the water was flowing seaward fast and some of them went out to observe. What do you think happened next? The people on the open beach drowned when the crest of the wave came up the beach.

Large tsunami in the Indian Ocean and more recently Japan have killed hundreds of thousands of people in recent years. The west coast is vulnerable to tsunami since it sits on the Pacific Ring of Fire. Scientists are trying to learn everything they can about predicting tsunamis before a massive one strikes a little closer to home.

Although most places around the Indian Ocean did not have warning systems in 2005, there is a tsunami warning system in that region now. Tsunami warning systems have been placed in most locations where tsunami are possible.

These awe-inspiring waves are typically caused by large, undersea earthquakesat tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly, it displaces the water above it and launches the rolling waves that will become a tsunami.

Most tsunamis–about 80 percent–happen within the Pacific Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.

Tsunamis may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

Tsunamis race across the sea at up to 500 miles (805 kilometers) an hour—about as fast as a jet airplane. At that pace, they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.

Source: National Geographic

Are tsunamis measured on a scale similar to those of tornadoes and hurricanes?

There is a tsunami intensity scale, although it is not used much anymore. Nowadays, tsunamis are usually described by their heights at the shore and the maximum runup of the tsunami waves on the land.

Authority: Dr. Frank Gonzalez, NOAA Center for Tsunami Research

What is the current tool used to detect tsunamis?

Tsunamis are detected by open-ocean tsunami buoys and by coastal tide gages. These instruments report their information in real-time to tsunami warning centers (one center in Alaska, another in Hawaii, and a third to be installed soon in Puerto Rico).

Besides the direct observations, the amplitudes of tsunamis are also estimated from the size and type of earthquake that may have generated them. The warning centers receive earthquake (seismic) data from many sites as well as analyses of this information from earthquake centers. Since seismic waves travel much faster than tsunamis, the earthquake information is often available hours before the tsunamis are able to travel across the ocean. This is much help for people near the earthquake, however, since the local tsunami is there often within minutes. It is that easy to estimate the strength of a tsunami from the character of an earthquake. Hence, there are false alarms. It is hoped that the direct tsunami observations will reduce the number of these.

Local tsunamis are also generated by underwater landslides and volcanoes. There is some research being done to better understand these types of tsunamis and to develop early detection methods for them.

Authority: Dr. Frank Gonzalez, NOAA Center for Tsunami Research