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A ship at sea doesn't notice a tsunami passing beneath it. The wave might be half a metre tall and 200 kilometres wide — so gradual that it registers as barely a ripple. Then it hits shallow water near shore and everything changes. The wave slows, compresses, and surges upward, sometimes to heights of 30 metres or more. By the time you can see it, there is no time left to run.
Tsunamis are among the most misunderstood natural phenomena on Earth. They are not tidal waves — they have nothing to do with tides. They are not one wave but a series. And the earthquake that generates them doesn't have to be close to be deadly. The 2011 Tōhoku tsunami killed people in Japan within minutes. The same wave killed people in California hours later. Understanding how they work is the first step toward surviving one.
Most tsunamis are generated by large undersea earthquakes — specifically, by vertical displacement of the ocean floor. When a subduction zone earthquake occurs, one tectonic plate is suddenly forced upward by several metres along a rupture zone that can stretch hundreds of kilometres. The ocean floor moves. The water column above it moves with it, instantaneously. That displaced water has to go somewhere, and it radiates outward in all directions as a series of long, fast waves.
Not every undersea earthquake generates a tsunami. The key factors are:
Wave height and wave energy are separate things. In deep water, a tsunami's energy is spread across the entire depth of the ocean — sometimes 4,000 metres or more. That energy manifests as a very long, very fast wave with a small surface height. A wavelength of 200 km and a height of 0.5 m is imperceptible on the open ocean, especially compared to regular wind-driven waves of similar height but wavelengths measured in tens of metres.
As the tsunami approaches shore and the ocean gets shallower, the wave's speed decreases dramatically — from 800 km/h to perhaps 50 km/h at the coast. But the energy has to go somewhere. The wave compresses: its wavelength shortens and its height amplifies. This effect, called shoaling, is why a modest open-ocean wave can stack into a wall of water at the shoreline. The ratio of amplification depends on how steeply the seafloor rises. Gradually sloping coastlines can focus energy violently; harbours and bays can act as resonance chambers, amplifying wave heights even further.
Tsunamis almost always arrive as a series of waves, not a single one. The waves are separated by intervals ranging from a few minutes to over an hour, depending on the length of the rupture zone and the distance from the source. The first wave is frequently not the largest — sometimes it's the third or fourth that does the most damage.
One of the most dangerous and counterintuitive effects is the drawback. Just before a tsunami wave arrives, the ocean may visibly recede — retreating hundreds of metres from the shore and exposing the seafloor in a way that looks startling and strange. This is the trough of the wave arriving before the crest. People sometimes walk out to investigate the exposed seabed. They have minutes, at most, before the wave follows.
The Pacific Tsunami Warning Center (PTWC), operated by NOAA in Hawaii, monitors seismic activity across the Pacific basin around the clock. When a qualifying earthquake occurs, the centre analyses its magnitude, location, depth, and fault type to assess tsunami potential. Buoys equipped with pressure sensors on the seafloor detect passing waves and transmit data in real time, allowing forecasters to update warnings as the wave propagates.
For distant tsunamis — those generated thousands of kilometres away — warning systems can provide hours of notice, enough to evacuate coastlines. For near-field tsunamis generated just offshore, the window can be as short as two to five minutes. No warning system can overcome that physics. In those cases, preparedness, public education, and vertical evacuation structures — purpose-built towers people can climb when there is no time to reach high ground — are the only defences.
Japan has invested heavily in tsunami defences since 2011: seawalls up to 15 metres tall along vulnerable coastlines, automated gate systems for harbours, evacuation towers in coastal communities, and mandatory tsunami education in schools. Whether those walls will be enough for the next great event remains an open question.
The distance a tsunami travels inland is called the run-up, and it can be far greater than most people imagine. The 2011 Tōhoku tsunami reached up to 10 kilometres inland in some areas of Japan's Tohoku coast. The 1960 Valdivia tsunami, which crossed the entire Pacific, still had enough energy to push 10 metres up onto the coasts of Hawaii and Japan more than 15 hours after the earthquake.
Elevation is the single most important factor in survival. Even a few metres of height can make the difference between safety and being swept away. Coastal communities in high-risk zones should know their evacuation routes instinctively — and those routes should lead uphill, not along the coast.
When you open the detail panel for any earthquake on Tremr, one of the data points shown is tsunami status — pulled directly from the USGS feed, which reflects alerts issued by NOAA and regional tsunami warning centres. If a tsunami warning is active for an event, it will appear in the panel immediately.
Large undersea earthquakes along the Ring of Fire — particularly in the subduction zones off Japan, Chile, Alaska, and the Pacific Northwest — carry the highest tsunami potential. These are the regions worth watching most closely on the map, not just for the shaking they cause on land, but for what they might be doing to the ocean above them.