A tsunami is a series of waves, or a “wave train,” generated by the sudden displacement of a large volume of water, typically caused by a submarine earthquake, landslide, or volcanic eruption. Unlike wind-driven waves, a tsunami propagates through the entire water column, carrying immense energy across ocean basins. The height of a tsunami is highly variable, changing dramatically as it travels from the deep ocean to the nearshore environment.
Tsunami Height in the Deep Ocean
In the deep ocean, where water depth is typically greater than 600 feet, a tsunami is often barely noticeable and poses no threat to ships. The wave height, measured from crest to trough, is frequently less than one meter (about three feet) and sometimes only a few centimeters high. This small vertical size means a mariner at sea would likely not detect a tsunami passing beneath the hull.
The wave’s energy is spread over a massive area because its wavelength, the distance between successive wave crests, can be hundreds of miles long. Since the speed of a tsunami is directly related to water depth, it can travel incredibly fast in the deep ocean, often exceeding 500 miles per hour. The wave loses very little energy as it travels, preserving its destructive potential upon reaching a coastline.
Coastal Amplification and Run Up
The small, fast-moving wave transforms completely when it approaches the coast through a process known as shoaling. As the tsunami enters shallow water, friction with the rising seabed causes the wave’s speed to decrease dramatically, slowing to about 20 to 30 miles per hour. This rapid decrease in speed forces the wave’s energy to be concentrated and pushed upward, leading to a significant increase in wave height.
This coastal amplification results in the destructive wave that reaches the shore, but the term “wave height” is often less important than “run-up.” Wave height refers to the measurement from crest to trough as the wave breaks or comes ashore. Run-up is the most critical measurement for impact, representing the maximum vertical height the water reaches above the normal sea level on land.
The run-up determines the extent of inland flooding and damage, even though a tsunami may manifest as a fast-rising tide rather than a classic breaking curl. Run-up measurements commonly range from 10 to 30 feet in significant events. In rare instances, especially close to the source, the maximum run-up height can exceed 100 feet as the water surges inland.
Geographic Factors Affecting Maximum Height
The final height and impact of a tsunami are heavily modified by local geographic features, explaining why run-up varies widely between nearby locations. The nearshore bathymetry—the shape and depth of the seafloor—is a significant variable. A steeply sloped offshore area causes less dramatic shoaling and less run-up, as the wave energy is not compressed gradually.
A gently sloping seabed allows the shoaling process to occur over a longer distance, focusing the wave energy more efficiently and leading to a higher run-up. While the magnitude of the initial event sets the total energy, local bathymetry determines how that energy is translated into vertical height.
Coastal geometry also plays a substantial role in localized wave amplification. Bays, harbors, and inlets can act as funnels, concentrating incoming wave energy and resulting in significantly higher run-up than on open coastlines. Conversely, offshore features like coral reefs can dissipate the wave’s energy, offering protection and reducing the maximum height reached on the shore.