Earthquakes and Tsunamis

Most earthquakes generate no tsunami at all. The specific circumstances required are narrow, which is why hundreds of offshore earthquakes each year produce nothing while a handful produce catastrophes.

What it takes to generate a tsunami

A tsunami is generated when a large volume of water is displaced vertically. The energy source is not the shaking itself but the physical deformation of the sea floor lifting or dropping the entire water column above it.

Four conditions generally need to be satisfied together:

The earthquake must be submarine. An inland earthquake cannot displace ocean water directly.

It must be shallow. As covered in the depth guide, deformation from a deep rupture is smeared out over such a wide area by the time it reaches the sea floor that vertical displacement becomes negligible. Warning criteria typically require depths under about 70 km.

It must be large enough. Below roughly magnitude 7.5 the sea-floor displacement is usually too small for an ocean-crossing tsunami, though smaller events can still produce dangerous local waves, particularly if they trigger a submarine landslide.

The fault motion must be vertical. This is the condition most often overlooked. A thrust fault pushes one block up over another, lifting the sea floor — ideal for tsunami generation. A strike-slip fault moves blocks horizontally past each other, displacing very little water vertically. This is why some very large strike-slip earthquakes produce surprisingly modest tsunamis.

Tsunamis can also be generated without any earthquake at all, by submarine landslides, volcanic collapses or eruptions, and very rarely by meteorite impact. Landslide-generated tsunamis can produce extreme local run-up heights but usually dissipate faster with distance than earthquake-generated ones.

How tsunami waves behave

A tsunami behaves nothing like an ordinary wind wave, and the differences explain why they are so dangerous.

Wind waves are a surface phenomenon; the water motion dies out within tens of metres of depth. A tsunami moves the entire water column, from surface to sea floor. That is why it carries so much energy and why it does not simply dissipate as it crosses an ocean.

In deep water, a tsunami may have a wave height of less than a metre but a wavelength of hundreds of kilometres. A ship in open ocean will pass over one without noticing. Its speed depends on water depth — roughly the square root of gravity times depth — which works out to 700–800 km/h over a 4,000 m deep ocean basin.

As it approaches shore the water shallows, and the wave slows. But the energy has to go somewhere, so the wave compresses horizontally and grows vertically. This is shoaling, and it converts an imperceptible deep-ocean swell into a wall of water at the coastline.

Two further characteristics matter for survival. First, a tsunami is not a single wave but a series, and the first is frequently not the largest. Waves can continue arriving for many hours. Second, because of the enormous wavelength, a tsunami does not break and retreat like a normal wave — it arrives more like a rapidly rising flood that keeps coming, then drains back out with tremendous force, carrying debris and people with it.

Natural warning signs

If you are near a coast, three natural signs should trigger immediate evacuation without waiting for any official message.

Strong or long shaking. If an earthquake is strong enough to make standing difficult, or if shaking continues for more than about 20 seconds, assume a tsunami is possible.

Sudden withdrawal of the sea. If water recedes rapidly, exposing sea floor and reef that is normally submerged, a trough is arriving before a crest. The crest will follow within minutes.

A loud roar from the ocean, often described as sounding like a train or aircraft.

For a locally generated tsunami you may have only minutes. Official warnings, however well engineered, cannot beat physics at close range.

Warning systems and alert levels

International warning centres — including the Pacific Tsunami Warning Center and the National Tsunami Warning Center, alongside regional systems in the Indian Ocean, Mediterranean and elsewhere — monitor seismic networks continuously.

The process runs in stages. Seismic data gives location, depth and magnitude within minutes, allowing an initial assessment based on the criteria above. That assessment is provisional, because seismic data alone cannot confirm whether water actually moved.

Confirmation comes from DART buoys (Deep-ocean Assessment and Reporting of Tsunamis), which use sea-floor pressure sensors to detect the passing wave in open ocean, and from coastal tide gauges. Once a tsunami is measured, models predict arrival times and run-up heights along threatened coastlines, and alerts are upgraded, downgraded or cancelled accordingly.

The alert levels have distinct meanings:

• Information statement — an earthquake occurred; no tsunami threat, or the event is far away and being evaluated.
• Watch — a tsunami is possible; stay alert for updates.
• Advisory — strong currents and dangerous waves are expected at the shoreline. Stay out of the water and away from beaches and harbours.
• Warning — significant inundation is expected or already occurring. Move inland or to high ground immediately.

An advisory is frequently misread as minor. It is not: tsunami currents in harbours can be lethal to swimmers and can destroy boats and docks even when no dramatic inundation occurs.

After the warning

Do not return to the coast until authorities declare it safe. Later waves in a series are often larger than the first, and dangerous currents persist long after the visible waves subside. Debris, damaged infrastructure and contaminated water make coastal areas hazardous well after the water level normalises.