A tsunami is a powerful series of waves generated by the rapid displacement of a large volume of water, typically caused by geologic activity on the ocean floor. Understanding the sequence of events that unfold before the wave strikes land is crucial for coastal communities. The process begins with a sudden shift deep beneath the ocean surface. This shift triggers both technological and natural warning responses that precede the destructive arrival of the wave train.
The Geological Trigger
The vast majority of destructive tsunamis originate from large, shallow earthquakes in subduction zones. These are locations where one tectonic plate is forced beneath a neighboring plate, causing immense stress to build up. When the overriding plate suddenly snaps upward, this vertical movement of the seabed displaces the entire column of water above it.
This vertical shift, caused by a thrust or reverse fault motion, differentiates tsunami-generating earthquakes from more common ones. Earthquakes involving primarily horizontal, or strike-slip, movement generally do not displace enough water to create a significant tsunami. The sudden raising or lowering of the seafloor acts like a giant paddle, pushing the ocean water out into a propagating series of waves.
Other, less common geological events can also trigger tsunamis, though these typically create more localized waves. Underwater landslides, sometimes triggered by an earthquake, can rapidly slump sediment down a continental slope, pushing the water column ahead of the debris. Similarly, the collapse of a volcanic flank or a submarine explosion can displace enough water to generate a wave train.
Official Tsunami Warning Protocols
The first indication of a potential tsunami comes from a global network of seismic monitoring stations. Since seismic waves travel much faster than tsunami waves, warning centers quickly calculate an earthquake’s location, magnitude, and depth. This initial data determines if the event occurred in a location and with a force likely to generate a tsunami.
To confirm the threat and provide accurate forecasts, scientists rely on the Deep-ocean Assessment and Reporting of Tsunamis (DART) network. This system uses pressure sensors placed on the ocean floor to detect the slight increase in pressure as a tsunami wave passes overhead. The data is transmitted to a surface buoy, which relays the information via satellite to warning centers.
Based on this combined information, warning centers issue a sequence of alerts to local officials and the public:
- A Tsunami Information Statement informs the public that an event occurred but poses no widespread threat.
- A Tsunami Watch suggests a potential threat exists and requires preparation.
- A Tsunami Advisory indicates the threat of dangerous currents and waves is present.
- A Tsunami Warning means a destructive tsunami with widespread flooding is imminent or occurring, requiring immediate evacuation.
Natural Indicators of Danger
For those close to the source, the immediate ground shaking from the earthquake serves as the first natural warning. If you are near the coast and feel an earthquake strong enough to make standing difficult, or if the shaking lasts for 20 seconds or more, a tsunami may arrive within minutes. In such cases, there may not be time for an official warning.
The most dramatic natural sign is the unusual and rapid recession of the ocean, often called the “draw-back.” A tsunami is a wave train with both a crest (high point) and a trough (low point). If the trough arrives first, it acts like a giant suction, pulling the water far away from the shore and exposing the seabed.
This exposed seabed can extend for hundreds of meters and should be interpreted as an immediate, life-threatening signal. The water will return with the wave’s crest, often within minutes, as a massive surge or a rapidly rising tide. Survivors often describe an approaching tsunami being accompanied by a loud, unusual sound, comparable to the noise of a freight train or a jet engine.
Taking Immediate Safety Action
Upon receiving an official Tsunami Warning or observing natural signs, immediate movement to safety is the priority. Do not wait for further instructions or attempt to confirm the threat; personal safety depends on speed and distance from the shoreline. The goal is to move to an area that is both high and far from the coast.
Evacuation guidelines recommend moving at least 100 feet above sea level or traveling two miles inland, away from the water. If you cannot reach the recommended elevation or distance, go as high and as far as possible. If evacuation is impossible, seek refuge on the fourth floor or higher of a structurally sound, reinforced concrete building as a last resort.
A tsunami is a series of surges that can arrive minutes to hours apart. The first wave is frequently not the largest or most destructive, and the danger period can last for many hours. Do not return to the coast until local authorities have officially announced that the threat has passed.