A tsunami is a series of ocean waves caused by a large and sudden displacement of water, most often triggered by an underwater earthquake. These forces generate waves that radiate outward across entire ocean basins. Outrunning a tsunami is not possible.
The Unseen Speed of a Tsunami
Tsunamis exhibit high speeds across the open ocean, often traveling as fast as a jet plane. In deep water, they can reach speeds exceeding 500 miles per hour (800 kilometers per hour), allowing them to cross vast distances in less than a day. As a tsunami approaches shallower coastal waters, its speed significantly decreases, slowing to about 20 to 30 miles per hour. This reduction in speed causes the wave to gain great height and power, a process known as shoaling.
A tsunami that might be small in the deep ocean can grow to a great height (over 100 feet) as it nears the shore. This transformation concentrates a large amount of energy, turning what appears as a gentle swell into a devastating force upon landfall. The wave’s behavior as a shallow-water wave, even in the deep ocean, is due to its long wavelength, which can span hundreds of miles.
Human Limits Against Nature’s Force
Comparing human speed to a tsunami’s speed at landfall highlights the impossibility of outrunning such an event. An average person’s running speed is around 8 to 9 miles per hour, while even elite sprinters reach peak speeds of 23 to 28 miles per hour. When a tsunami reaches the coast, it can still be moving at speeds of 20 to 30 miles per hour, which is faster than most people can run.
The danger from a tsunami extends beyond its speed. It involves a large volume of water and debris. This strong surge can sweep away individuals, cars, buildings, and other infrastructure. Tsunamis are not single waves; they often consist of a series, or wave train, arriving minutes or hours apart. The initial wave may not be the largest, so danger can persist.
Immediate Safety Actions
Since outrunning a tsunami is not an option, immediate action is necessary for survival. In a coastal area, evacuate immediately to higher ground and as far inland as possible. Aim for at least 100 feet above sea level or one mile inland.
Evacuating on foot is the most reliable method, as roads may become impassable due to damage, traffic congestion, or flooding. Avoid rivers and streams, as tsunami waves can travel rapidly up these waterways, catching people by surprise. If reaching high ground is not feasible, seeking refuge on the fourth floor or higher of a sturdy, reinforced concrete building may offer some protection. Remain in a safe location until authorities issue an “all clear” message, as the danger can persist for hours with subsequent waves. Even a few inches of fast-moving water can be strong enough to knock adults off their feet due to strong currents and debris.
Heeding Nature’s Warnings
Recognizing natural warning signs can provide time to react, even before official alerts are issued. One primary indicator is a strong earthquake felt while on the coast, especially if the shaking lasts for a long duration. Such an earthquake could generate a tsunami that arrives within minutes.
Another natural sign is a sudden recession of the ocean from the coastline, exposing the seafloor far beyond a normal low tide. This “drawback” indicates that the trough of a tsunami wave is approaching before the crest arrives. A loud roar from the ocean, often described as sounding like a train or jet engine, is an auditory warning sign. If any of these natural warnings are observed, act immediately and move to safety without waiting for official instructions.