Tukuk-OSA storm wave moves only water near the surface; a tsunami moves the entire water column at once. An undersea earthquake that vertically faults the seafloor displaces a slab of ocean kilometres high and hundreds of kilometres wide, and gravity flattens the bump by flinging it outward at speeds near 700–800 km/h — a jetliner’s cruise, in water only 4 km deep. In the open ocean the tsunami is almost invisible: a gentle slope tens of centimetres high spread over 100–200 km of wavelength. Nobody feels it from a ship.
Danger builds as it shoals: the wave drags on the bottom, slows to tens of km/h, and its energy compresses into a wall metres to tens of metres tall. A tsunami is also a series of waves arriving minutes apart — the first is often not the biggest, and the sea pulling strangely far out from the shore is a classic premonition of surge. The fatal mistake is “wait and see”, because the natural warning signs are the warning.
Physical signs that demand immediate movement to high ground (20–30 m or 2–3 km inland): a strong or long-shaking earthquake near the coast, an unusual roar from the sea, or sudden drain-out of the surf. The 2004 Indian Ocean disaster killed around 230,000 people; the one village famously spared, on Indonesia’s Simeulue island, survived because oral history had taught its people to run to the hills when the sea withdrew — after the 1907 tsunami the elders had turned into story. Warning systems exist now across the Pacific and Indian oceans; they bought their first test in 1960 after the Chile earthquake’s tsunami crossed an ocean unannounced and killed in Japan hours later.