Science & Tech

Tsunamis 101

By Rohan D. ’25

On Saturday, January 15th, the Hunga Tonga-Hunga Ha’apai volcano erupted. This volcano is situated on an uninhabited island of Tongatapu, Tonga’s main island. According to the Tonga Geological Services, ash and plumes from the explosive eruption reached about 150 miles in width and more than 12 miles above sea level; for reference, Mount Everest is around 5.5 miles above sea level. However, because this volcanic eruption occurred so close to Tonga, ash rained down on the archipelago and possibly contaminated the drinking water. According to NPR News, “Residents were advised to cover water reservoirs and to check their roofs for ash before reconnecting rainwater systems.”. Because of the magnitude of the explosion, geologists could see the eruption easily on satellite imagery. It was followed by another smaller volcanic explosion. In addition, authorities across the Pacific issued tsunami warnings, stating that “unpredictable surges” and “dangerous rips, waves, and strong ocean currents” were possible. The next day, New Zealand’s National Emergency Management Agency warned, “strong currents and surges can injure and drown people. There is a danger to swimmers, surfers, people fishing, small boats, and anyone in or near the water close to shore.” In fact, according to the New Zealand Herald, “a 6-foot surge in Tutukaka, a town some 85 miles north of Auckland, New Zealand, damaged a harbor and about 30 boats docked there.” On the other side of the Pacific Ocean, in the United States, the US National Weather Service reported that “the highest surge was recorded in Port San Luis, Calif., where the water reached more than 4 feet over normal levels.”

Diagram of Tsunami From Generation to Landfall | Credit: Encyclopædia Britannica, Inc.

Tsunamis are larger than regular waves that one finds at a beach. While normal ocean waves are generated by wind or tides (because of the gravitational pull of the Moon and the Sun), a tsunami is formed by the displacement of water by a large event, such as earthquakes, volcanic eruptions, submarine landslides, and other underwater explosions. Tsunamis are also different than normal ocean waves because their wavelengths are much shorter; in an open ocean, the wavelength (distance between waves) of the tsunami is high, the amplitude (height of waves) is low, and the wave speed is high. As the tsunami encounters the rising seafloor and the coast, the wavelength decreases, the amplitude increases, and the wave is moving relatively slowly. This means that when tsunamis break on the coast, the energy from the explosion in the open water is translated into many towering waves that can travel across oceans. 

Tsunamis can cause destruction in two ways: the force generated by the wave traveling at high speed inland, as well as the damaging power of a large volume of water draining off the land back to sea and sucking lots of debris with it. Right before a tsunami, a drawback of hundreds of meters will occur as the waterline recedes dramatically, exposing normally submerged areas. This is a telltale sign of tsunamis, so one should try to get to high ground as fast as possible instead of exploring those unsubmerged areas. These are hazards even when the waves do not appear to be large. “While everyday wind waves have a wavelength (from crest to crest) of about 100 meters (330 ft) and a height of roughly 2 meters (6.6 ft), a tsunami in the deep ocean has a much larger wavelength of up to 200 kilometers (120 mi). Such a wave travels at well over 800 kilometers per hour (500 mph), but owing to the enormous wavelength the wave oscillation at any given point takes 20 or 30 minutes to complete a cycle and has an amplitude of only about 1 meter (3.3 ft)” (Earth Science Australia). This means that tsunamis carry a lot of energy, which is translated into pressure. According to the New York Times, tsunamis have a pressure of 45,000 pounds per square foot, which, for the average human, is like having 100 tons of force pressing one down.

Digitally altered image of tsunami waves sweeping over city (digital alteration; natural disaster)
Tsunami Making Landfall | Credit: Christophe Fouquin/Fotolia

If you ever find yourself seeing the water recede hundreds of feet and a tidal wave barreling towards you, there are a few things you should do. First, since tsunamis usually originate from earthquakes, you should drop, cover, and hold to protect yourself from the earthquake. Also, listen to officials and emergency alerts. However, if you receive no alerts, evacuate instead of staying put. When the earthquake has subsided, seek high ground and move as far inland as possible. If you are in a boat, head out to sea. Hopefully, you never find yourself threatened by a tsunami, but if you do, this article as well as government websites should provide you with the necessary information to survive.

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