A tsunami is a series of powerful ocean waves caused by large-scale disturbances that displace a significant volume of water. Unlike wind waves, tsunamis can travel across entire ocean basins and gain immense height upon reaching coastal areas. Its height varies significantly with location and conditions.
Tsunami Formation and Characteristics
Most tsunamis originate from large underwater earthquakes, often in subduction zones where plates slide. These seismic events cause the seafloor to uplift or subside, displacing water and generating the initial wave. Other sources include underwater landslides and volcanic eruptions, which also trigger massive water displacements.
In the open ocean, a tsunami behaves differently from a regular surface wave. It has an exceptionally long wavelength, sometimes hundreds of kilometers, and travels at speeds exceeding 800 kilometers per hour. Its height in the deep ocean is often only tens of centimeters, making it imperceptible to ships. This allows tsunamis to cross vast oceanic distances without significant energy loss.
Measuring Tsunami Height
Tsunami height is measured differently in the open ocean versus its impact on land. While negligible in the deep ocean, its destructive potential becomes apparent near the coast. Open ocean “tsunami wave height” refers to the vertical distance from crest to trough.
The more relevant measure for assessing a tsunami’s impact is “run-up” or “inundation height.” Run-up is defined as the maximum vertical height above a reference sea level that the water reaches inland. This captures the combined effect of wave energy, local topography, and tidal conditions, indicating the tsunami’s destructive power.
Factors Affecting Coastal Run-up
Geological and geographical factors influence a tsunami’s coastal run-up. Seafloor topography is one factor; shallowing depths cause the tsunami’s speed to decrease and its height to increase through shoaling. Underwater ridges or canyons can focus or disperse wave energy, modifying height.
Coastline shape plays a substantial role in determining run-up. Bays, harbors, and narrow inlets can act like funnels, concentrating the tsunami’s energy and amplifying its height as the wave is squeezed into a smaller area. Conversely, broad, open coastlines may experience less run-up.
Land elevation and slope behind the coastline dictate how far and high water travels inland. Steep cliffs or high elevations limit inundation distance, while low-lying, gently sloping areas allow water to spread farther inland, leading to widespread flooding. Tsunami source characteristics, like earthquake magnitude, depth, and seafloor rupture direction, influence initial wave energy and direction, affecting potential run-up heights.
Record-Breaking Tsunami Heights
History provides examples of exceptionally high tsunami run-up measurements. The highest ever recorded occurred on July 9, 1958, in Lituya Bay, Alaska. Triggered by a massive landslide into the narrow fjord, the resulting megatsunami surged to a run-up height of 524 meters (1,719 feet) on the opposite mountainside. This event demonstrates how localized geological factors can produce waves far taller than typical earthquake-generated tsunamis.
The 2011 Tohoku tsunami in Japan was caused by a magnitude 9.1 earthquake. Specific locations experienced extreme run-up heights; for instance, in Miyako, Iwate Prefecture, the run-up reached an estimated 40.5 meters (133 feet) above sea level. This highlighted the vulnerability of coastal communities to powerful tsunamis, even with advanced warning systems.
The 2004 Indian Ocean tsunami, generated by a magnitude 9.1-9.3 earthquake off Sumatra, was one of the deadliest natural disasters in modern history. While it impacted 14 countries, the highest run-up was recorded on the western coast of northern Sumatra, near Banda Aceh, reaching 51 meters (167 feet) in some areas. This event showed the importance of international cooperation and early warning systems for regions susceptible to transoceanic tsunamis.