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Seismic Activity
According to the National Oceanic and Atmospheric Administration, most tsunamis are caused by earthquakes that happen at sea or along coastlines. The Earth's surface, including its ocean floors, is made up of tectonic plates that move over time. When ocean plates slide beneath continental plates, they cause earthquakes that rapidly force up sections of sea floor. This uplift displaces seawater, forcing it up and creating fast-moving ripples along the ocean's surface. These are the beginnings of many tsunami waves.
Water Displacement
Not all tsunamis begin underwater, or are due to earthquakes. Anything that displaces large amounts of water can produce tsunami waves. Volcanoes cause about 5 percent of tsunamis. For example, underwater volcanoes can erupt, or coastal volcanoes can propel rubble into coastal water, creating fast-moving waves. Landslides can also displace water; when earthquakes cause landslides, they amplify tsunami effects. A large asteroid or meteor striking at sea could send out tsunami waves, like the ripples from throwing stones into water on a much larger scale. Glaciers have even been observed creating tsunami waves. They break off from ice shelves in irregular shapes and sizes, often capsizing to settle into stable positions, creating destructive waves.
Giant Waves
Tsunami waves are small at sea, typically under 1 meter (3 feet) tall, but traveling at over 700 kilometers per hour (435 miles per hour). Like all waves, they travel in rising crests and falling troughs; from land, the first sign of an impending tsunami is a sudden high or low tide, depending on whether a crest or trough hits first. Tsunami waves slow in shallow coastal water, but their vast energy causes them to grow rapidly. They can reach heights of over 100 feet before crashing into coastlines with destructive force.
Guarding Against Tsunamis
In the U.S., the Pacific Tsunami Warning Center and National Tsunami Warning Center work to detect tsunamis and warn the public of them. For example, a network of Deep-ocean Assessment and Reporting of Tsunamis (DART II) monitors identify sea level fluctuations in order to model developing tsunami waves. In the event of potential tsunami-causing events like underwater earthquakes, they can be placed on high alert to increase detection. Through a combination of early warnings, community disaster planning, and public education systems, the danger to human lives can be lessened. Sea walls can repel tsunamis, but they can be overwhelmed by larger-than-anticipated tsunamis, limiting their effectiveness.