A tsunami is a series of ocean waves generated by the sudden displacement of a massive volume of water, typically resulting from an underwater earthquake, landslide, or volcanic eruption. Unlike common, wind-driven surface waves, a tsunami involves the movement of the entire water column, from the ocean floor to the surface. It is often mislabeled as a “tidal wave,” but it is unrelated to the gravitational forces that create tides. A tsunami can travel across vast distances virtually unnoticed, only revealing its destructive power as it nears the coast.
Tsunami Speed and Wavelength in the Open Ocean
The speed of a tsunami in the deep ocean is immense, yet its vertical height is remarkably small. In water depths averaging 4,000 meters (13,000 feet), the wave can travel up to 800 kilometers per hour (500 mph), a velocity comparable to a commercial jet aircraft. This extreme speed is governed by the water depth, following the physical principle that wave speed is proportional to the square root of the depth.
Despite this rapid movement, the tsunami wave height in the open ocean is often less than one meter (three feet), making it virtually imperceptible to ships. This small height is coupled with an extraordinarily long wavelength—the distance between successive wave crests—which can stretch up to several hundred kilometers. Because the wavelength is vastly greater than the ocean depth, a tsunami behaves as a shallow-water wave even in the deepest parts of the ocean. This allows the wave to retain its energy over thousands of kilometers, enabling it to cross entire oceans in less than a day.
Factors Determining Tsunami Wave Height
The benign appearance of a tsunami changes dramatically as it approaches land due to shoaling. As the wave enters shallower water over the continental shelf, friction with the seabed causes the front of the wave to slow down. Since the energy of the wave train is conserved, the water compresses and piles up, leading to a dramatic increase in wave height.
The tsunami’s speed can drop from over 800 km/h in the deep ocean to 30 to 50 km/h (20 to 30 mph) near the shore. This deceleration translates directly into vertical amplification. The wave may appear not as a typical breaking surf wave but as a rapidly rising tide or a turbulent wall of water, known as a bore. The ultimate destructive measure is the run-up.
Run-up is the maximum vertical elevation the water reaches above mean sea level as it surges inland. This measurement often exceeds the initial wave height because the water’s momentum carries it up slopes. Run-up from distant tsunamis may reach 15 meters (50 feet), while near-source tsunamis can exceed 30 meters (100 feet).
Local geographical features heavily influence the final run-up height, leading to variability over short coastal distances. Submarine topography, or bathymetry, dictates how the wave energy is focused or dispersed. Wide, shallow bays or funnel-shaped inlets can concentrate the energy, dramatically amplifying the wave and the resulting run-up.
Coastal topography also plays a role; cliffs and steep shorelines force the water to climb higher vertically, while gentle, sloping beaches allow the water to travel farther inland horizontally. The highest recorded run-up was 525 meters (1,720 feet) in Lituya Bay, Alaska, in 1958, caused by a massive, local landslide. This event highlights how local factors can transform an energetic wave into an unprecedented surge.
Classifying Tsunami Magnitude
Scientists require a way to quantify the overall size and destructive potential of a tsunami event, independent of variable measurements taken at a single coastal location. Early methods focused on the average maximum wave height along a coastline, leading to intensity scales like the Soloviev-Imamura scale. Modern approaches attempt to capture the overall energy of the event rather than just localized effects.
The Tsunami Magnitude Scale (Mt) correlates the seismic energy of the generating earthquake with the amplitude of the tsunami waves recorded by gauges. However, a large earthquake does not always generate a large tsunami; smaller earthquakes or landslides can sometimes generate disproportionately large waves. This emphasizes the need to measure the energy transferred into the ocean, not just the magnitude of the seismic source.
Dedicated intensity scales, such as the 12-point Integrated Tsunami Intensity Scale (ITIS-2012), are used to classify the overall event based on observed effects on humans, the environment, and built structures. These scales provide a standardized way to compare historical events by focusing on the overall destructive footprint and the energy flux of the entire tsunami. This classification helps in understanding the scale of the hazard and informing warning systems.