A tsunami is a series of ocean waves generated by the displacement of an immense volume of water. Unlike wind-driven surface waves, a tsunami involves the movement of the entire water column, from the surface to the seafloor. This unique characteristic means its destructive potential is not measured by its height in the open ocean, but by how that height is amplified near the coast.
The Seismic Origins of Tsunami Scale
The most powerful and widespread tsunamis are born from megathrust earthquakes in subduction zones, where one tectonic plate is forced beneath another. This process builds up stress over long periods.
When the stress is suddenly released, the overriding plate snaps upward, causing a large, rapid vertical displacement of the seafloor. This abrupt shift lifts or drops the entire water column above it, generating the tsunami wave.
While megathrust earthquakes are the primary drivers of basin-wide tsunamis, other events like underwater landslides and volcanic activity can also trigger waves. These secondary causes usually result in more localized tsunamis because they displace a smaller volume of water over a limited area. For a tsunami to pose a distant threat, the originating earthquake needs to exceed a magnitude of 8.0, ensuring a sufficiently large area of seafloor uplift.
Deep Ocean Velocity and Wavelength
In the deep ocean, a tsunami moves with incredible speed. The wave’s velocity is entirely dependent on the depth of the water it is traveling through. Over the average ocean depth of 4,000 meters, a tsunami can travel at speeds exceeding 500 miles per hour, comparable to the speed of a jetliner.
Despite this speed, the wave’s amplitude, or height, is remarkably small, often less than three feet. This low height, combined with its immense wavelength, is why tsunamis go unnoticed by ships in the open sea, appearing only as a slight swell. The wavelength, the distance from one wave crest to the next, can span up to 120 miles.
The wave is considered a “shallow water wave” even in the deepest parts of the ocean because its wavelength is much greater than the water depth. This characteristic allows the wave to propagate across entire ocean basins with minimal energy loss. The wave period, the time between successive crests, is also very long, ranging from a few minutes to over an hour.
Coastal Amplification and Runup Height
The wave’s characteristics change dramatically as it approaches a coastline in a process known as shoaling. When the wave moves from the deep ocean onto the shallower continental shelf, friction with the seafloor causes the front of the wave to slow down significantly. As the leading edge slows, the faster-moving water behind it continues to push forward, resulting in a massive compression of the wave.
This compression forces the wave’s energy into a much smaller volume, which translates directly into a rapid increase in wave height. The speed of the wave can drop from over 500 miles per hour to as low as 20 to 50 miles per hour near the shore. A wave that was less than three feet high in the open ocean can transform into a towering mass of water many tens of feet high.
The destructive measure of a tsunami at the coast is not just its wave height, but its runup height. Runup is the maximum vertical height the water reaches above sea level on the land. While a wave height of 30 feet is common for large events, the maximum runup height can be much greater, sometimes exceeding 100 feet in confined bays or fjords due to funneling effects. The local coastal topography and the slope of the seafloor play a significant role in determining how much a tsunami is amplified.
Detecting and Measuring Tsunami Size
Scientists quantify and predict the size of an incoming tsunami using a network of sophisticated instruments. The most important tool for early detection is the Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy system. These systems consist of a seafloor-anchored bottom pressure recorder (BPR) that measures tiny changes in water pressure caused by the passing wave.
The BPR transmits this data acoustically to a surface buoy, which then relays the information via satellite to Tsunami Warning Centers. The DART system measures the true amplitude and speed of the wave while it is still in the deep ocean. This data allows scientists to create refined forecasts of the wave’s size and arrival time at distant coastlines.
Closer to shore, coastal water-level stations and traditional tide gauges are used to confirm the tsunami’s arrival time and measure its height near land. By combining the deep-ocean data from DART with seismic information and coastal measurements, emergency services can issue timely and accurate warnings.