Tukuk-OSNot a Wave — a Displacement
Ordinary waves are wind rippling the surface. A tsunami is something else entirely: a huge volume of water lifted or pulled out of place all at once, so that the ocean tries to level itself out and that energy races across the basin. Most tsunamis are caused by undersea earthquakes at subduction zones, where one tectonic plate lurches vertically under another and the seafloor suddenly rises or drops by metres. The water above it has nowhere to go but sideways and outward — a wall of energy with the wavelength of a hundred kilometres. The same shaking that we measure with the earthquake magnitude and intensity scales can, if it is vertical and shallow and big enough, become a tsunami.
What Else Can Cause One
Earthquakes are the main trigger, but not the only one. A volcanic eruption or flank collapse can shove water aside (and send a pyroclastic surge into the sea); a submarine or coastal landslide — on land or under the sea — can displace water locally and violently; and a large meteorite impact would do the same, though this is rare. Importantly, weather never causes a tsunami. Storm surges and big seas are a completely different phenomenon, which is why "tidal wave," the old term, is doubly wrong: tsunamis have nothing to do with tides or with waves as we usually mean them.
Fast and Invisible at Sea, Slow and Enormous Ashore
In the deep ocean a tsunami travels at the speed of a jetliner — around 700 to 800 km/h — but its height may be only a few centimetres, spread over an enormous wavelength, so ships overhead often detect nothing. The physics changes near land. As the water shoals into shallow coastal shelves, the wave slows down and its energy compresses into a smaller depth, so its height grows — a process called shoaling. The single wave also becomes a long series of surges lasting minutes to hours. A classic warning sign is the sea suddenly drawing back far beyond the normal low-tide line, exposing the seabed, as water is sucked in to feed the approaching crest.
How Warnings Buy You Minutes
No one can predict the earthquake that will cause the next tsunami — but the earthquake itself can be detected in seconds. Networks of seismic stations plus deep-ocean pressure recorders called DART buoys confirm whether a wave has actually formed and where it is heading, letting warning centres estimate arrival times for distant coasts. Because a tsunami is generated by the shaking, the most reliable early warning near the source is the earthquake itself: if the ground shakes strongly and you are on the coast, move to high ground immediately rather than waiting for a siren. For those farther down the coast, minutes matter, and the rule taught in every tsunami-ready town is simple — go to higher ground or inland, stay there, and do not return until officials say the series of surges has ended.
Distance Is a Barrier and a Clock
Tsunami energy crosses entire oceans, so a quake in one basin can strike coasts thousands of kilometres away hours later — which is why the Pacific has run a warning system for decades and the Indian Ocean built one after the devastating 2004 event. The trade-off is that distance also buys time: energy dissipates as it spreads, so far-away shores usually face smaller, later surges than those near the source.