A tsunami is not a single tidal wave but a series of powerful, long-wavelength ocean waves generated by the sudden displacement of a massive volume of water, most commonly from a large underwater earthquake. This displacement creates a wave train that can travel across entire ocean basins, often unnoticed in the deep sea but gaining immense height and destructive force as it nears the shore. Survival chances are highly variable, moving from near certainty for those far inland to extremely low for those caught directly in the inundation zone without warning. Understanding the forces at play and knowing the immediate, precise actions to take are the two factors that most directly influence the probability of living through the event.
Key Variables That Determine Survival Rates
The probability of surviving a tsunami is largely determined by pre-existing geographic factors. Coastlines featuring a broad and shallow continental shelf tend to dissipate a significant portion of a tsunami’s energy through friction, offering a natural buffer against the full force of the waves. Conversely, coastlines where the ocean floor drops off steeply, such as those common around the Pacific Ring of Fire, provide little resistance, allowing the wave to maintain its speed and energy until it nears the immediate shore.
A tsunami is a series of waves, sometimes arriving hours apart, and the first wave is frequently not the largest or most damaging. The wave’s speed is directly related to water depth, traveling up to 550 miles per hour in the deep ocean. As it reaches shallower water, this “shoaling” effect causes the wave’s height to increase dramatically, transforming an imperceptible swell into a devastating rush of water at the coast.
Effective warning systems and communication networks significantly increase the chances of survival by buying evacuation time. Detection systems, such as Deep-ocean Assessment and Reporting of Tsunami (DART) buoys, monitor pressure changes on the seafloor to provide official alerts when a distant-source tsunami is generated. However, for tsunamis caused by local earthquakes near the coast, the time between the seismic event and landfall can be only minutes, rendering official warnings ineffective. In these cases, the ability of residents to recognize natural warning signs becomes the single most important factor.
Immediate, Life-Saving Actions During the Event
The immediate recognition of natural warnings provides the opportunity to self-evacuate, which is often the only way to survive a local-source tsunami. A strong earthquake that is difficult to stand up during, or a weak one that lasts for a minute or more, signals that a large displacement event may have occurred. Other signs include a rapid, unusual recession of the ocean water, exposing the seafloor, or a loud roar coming from the sea. If you feel a long or strong earthquake near the coast, you must immediately move.
Evacuation must be directed inland and upward, as quickly as possible, without waiting for any official alert. The safe zone is at least 100 feet (30 meters) above sea level, or one mile (1.6 kilometers) inland, and should be reached on foot to avoid traffic congestion. The danger is not over once the first wave passes, as succeeding waves often arrive with greater force and higher run-up heights. Authorities must issue an official “all clear” before anyone attempts to return to low-lying areas.
If elevation or distance is impossible to achieve, vertical evacuation to the upper floors of a structurally sound, reinforced concrete building offers the next best chance for survival. The building should have at least three stories to provide adequate refuge from the inundation zone. As a last resort if trapped in the open, climbing a sturdy tree or grasping onto floating debris can provide a temporary means of staying above the turbulent water. Securing a position of refuge is paramount, as the force of the water is capable of sweeping away debris and people.
Secondary Hazards and Post-Event Survival
Survival does not guarantee safety, as the post-tsunami environment presents numerous secondary hazards. The receding waters carry a large load of debris, including shattered timber, vehicles, and metal, which pose a risk of blunt force trauma and severe lacerations. People must remain vigilant against this swift-moving, debris-laden water.
Contaminated water is a long-term survival threat, as flooding breaches sewage lines and mixes with chemical spills. This inundation introduces pathogens that can lead to outbreaks of waterborne diseases like cholera and typhoid. Seawater intrusion can also contaminate freshwater sources, such as wells and aquifers, rendering them unusable for drinking without proper purification.
The loss of public utilities further complicates recovery and survival efforts. Damaged infrastructure makes movement treacherous and slows the arrival of medical and humanitarian aid. Key infrastructure failures include:
- Communication systems
- Power supplies
- Clean water supplies
- Collapsed bridges
- Downed power lines
It is necessary to wait for local authorities to declare the area safe. All drinking water sources must be tested for contamination, and any water that is not bottled should be boiled or treated before consumption.