What Causes A Tsunami?
QUICK ANSWER
Tsunamis are caused by sudden displacement of large amounts of water, usually from underwater earthquakes that shift the seafloor. Other causes include underwater volcanic eruptions, undersea landslides, coastal landslides into water, and rare meteorite impacts. The displaced water spreads outward as fast-moving waves that can cross entire oceans.
Tsunamis are among the most devastating natural hazards, capable of crossing entire oceans and devastating coastlines thousands of miles from their source. The underlying physics is straightforward: anything that suddenly displaces a large amount of ocean water can generate a tsunami. The most common cause is underwater earthquakes, but volcanic eruptions, landslides, and other events have all triggered destructive tsunamis throughout history.
How do earthquakes cause tsunamis?
About 80% of tsunamis are caused by underwater earthquakes, especially in subduction zones where the seafloor moves vertically during the quake. When a major subduction zone earthquake occurs, an enormous section of seafloor suddenly shifts upward or downward by feet or even tens of feet. This vertical displacement pushes or pulls the entire water column above it, creating waves that spread outward in all directions. The size of the tsunami depends on the size of the earthquake, the amount of vertical seafloor displacement, and the water depth at the source.
Can volcanoes cause tsunamis?
Yes, volcanoes can cause tsunamis through several mechanisms. Underwater volcanic eruptions can displace water directly when the eruption is violent enough. Volcanic flank collapses, where part of a volcano slides into the ocean, can generate massive tsunamis. The 1883 Krakatoa eruption created tsunamis that killed about 36,000 people, primarily caused by collapse of the volcano into the surrounding sea. Pyroclastic flows entering the ocean can also generate tsunamis. The 2018 Anak Krakatau eruption produced a deadly tsunami when part of the volcano collapsed into the sea, killing over 400 people.
How do landslides cause tsunamis?
Landslides into water bodies can produce powerful tsunamis. Coastal landslides where rock and soil slide into the ocean displace water dramatically. The 1958 Lituya Bay tsunami in Alaska, triggered by an earthquake-caused landslide, produced the highest tsunami wave ever measured at over 1,700 feet. Underwater landslides triggered by earthquakes or natural sediment instability can also produce tsunamis without surface evidence of the cause. Some tsunamis from past geological history were generated by enormous submarine landslides, including events that may have affected ancient coastlines significantly.
How are tsunamis different from regular waves?
Tsunamis are fundamentally different from wind-driven waves. Regular ocean waves are surface phenomena with a wavelength typically under 1,000 feet, periods of seconds, and energy concentrated near the surface. Tsunamis are full-water-column phenomena with wavelengths of tens to hundreds of miles, periods of 10-60 minutes, and energy distributed through the entire water depth. In the open ocean, a tsunami may be only a few feet tall and barely noticeable, but its enormous wavelength carries vast energy. As the tsunami reaches shore and water shallows, that energy compresses into much taller waves.
Tsunamis are caused by anything that suddenly displaces a large amount of ocean water, with underwater earthquakes being the most common cause (about 80% of tsunamis). Volcanic eruptions and flank collapses, undersea and coastal landslides, and rare meteorite impacts make up the rest. The fundamental physics is water displacement, while the size depends on the volume displaced and the speed of displacement.
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