A tsunami is a series of ocean waves caused by the sudden displacement of water, most often triggered by an underwater earthquake, but sometimes by landslides or volcanic activity. Unlike typical wind-driven waves that only affect the ocean’s surface, a tsunami involves the entire water column, from the seabed to the surface. This massive volume of moving water presents a scenario where outrunning the threat hinges entirely on speed and distance. Understanding the physics of these waves and recognizing the immediate warning signs are the only real defenses against this natural phenomenon.
The Physics of Tsunami Speed
The short answer to whether a person can outrun a tsunami is universally no, and the reason lies in its velocity. In the deep ocean, a tsunami can travel at speeds comparable to a commercial jetliner, often exceeding 500 miles per hour (800 kilometers per hour). At this speed, the wave is barely noticeable, often less than one meter high, due to the enormous depth beneath it.
The wave’s speed is a function of the water’s depth. As the tsunami approaches the coastline, the seabed rises, and the water depth decreases dramatically. This transition forces the wave to slow down, often to a speed between 20 and 30 miles per hour (32 to 48 kilometers per hour) as it reaches the shallow near-shore water.
However, even this reduced speed is faster than the average human can sprint, which is typically around 15 miles per hour. Running horizontally is a futile attempt against a force that can maintain a speed equivalent to a car moving through an urban area.
This process of slowing down and transforming near the shore is known as shoaling. As the wave’s speed decreases, its energy is compressed into a much smaller volume of water. This compression causes the wave height to increase dramatically, sometimes swelling from a barely perceptible bump in the deep ocean to a wave cresting over 30 feet high near the coast. The danger is not just the wave’s velocity but its immense volume and the force of the resulting inundation, which acts more like a rapidly rising, debris-filled flood than a typical breaking ocean wave.
Recognizing Natural Warning Signs
Since the speed of the wave makes a horizontal escape impossible once the surge is visible, time becomes the only factor that can save lives. Nature often provides clear, immediate warnings that precede an official alert.
The first sign is the seismic shaking itself. If you are near the coast and feel an earthquake that lasts for 20 seconds or more, a local tsunami may be imminent. Such a long-duration quake indicates a major displacement of the seafloor, which is the mechanism that generates the destructive waves.
Another sign is the loud, unusual sound that emanates from the ocean as the wave approaches. Eyewitnesses describe this sound as a deep, continuous roar, similar to that of a freight train or a jet aircraft. This noise is generated by the massive volume of water and air being churned and compressed as the wave enters shallow water. If this sound is heard, it means the wave is only moments away from impacting the shore.
A third warning is the phenomenon known as the ocean “drawdown.” This occurs when the leading edge of the tsunami is a trough, causing the water to recede rapidly and unusually far from the shoreline, exposing the seabed that is normally underwater. Seeing this exposed ocean floor is an immediate signal to move to higher ground. However, a drawdown does not always occur; the first wave can just as easily be a crest, leading to a sudden surge with no prior recession.
Immediate Survival Protocol
The moment any natural warning sign is observed, the only viable survival strategy is to seek elevation immediately. Do not wait for official sirens or confirmation, as a locally generated tsunami may arrive within minutes.
The directive is to move on foot, as roads may be damaged or quickly congested, and debris can make driving impossible. The goal is to reach high ground that is at least 100 feet (30 meters) above sea level, or to move a minimum of one to two miles inland, away from the coast. Every inch of elevation and every foot of horizontal distance gained can be the difference between survival and being caught in the inundation zone. Once you have reached a safe area, you must stay there.
If reaching high ground is not immediately possible due to flat terrain or insufficient time, “vertical evacuation” becomes the last resort. This involves moving to the highest floor of a sturdy, structurally sound building, ideally one constructed of reinforced concrete or steel frame. These types of structures are better equipped to withstand the lateral forces and debris impact of the water surge.
After the first wave passes, do not descend or return to the coast. Tsunamis are a series of waves, and the initial surge is often not the largest. Remain in the safe location until local officials issue an official “all clear” announcement, as dangerous follow-up waves can continue to arrive for eight hours or longer.