When an Earthquake Causes a Tsunami (and When It Doesn't)

When a big earthquake strikes near an ocean, the first question everyone asks is the same: is a tsunami coming? The answer is surprisingly often "no" — most earthquakes, even large ones, produce no significant wave at all. But the rare ones that do can cross an entire ocean and devastate coasts thousands of miles away. What separates the two is one of the clearest fusion stories on the map: it's not just about the earthquake, and not just about the sea, but about how the two connect.

A tsunami is the seafloor moving water

The key idea is simple. A tsunami is born when the ocean floor itself suddenly shifts vertically, lifting or dropping a vast volume of water in an instant. That displaced water spreads outward as a series of long, fast waves. So the question "will this quake make a tsunami?" really becomes "did this quake move the seafloor up or down, and over a big area?"

That points to a specific kind of earthquake. The great tsunami-makers are undersea megathrust quakes, where one tectonic plate is forced over another at a subduction zone, heaving the seabed upward over a huge expanse. These are the magnitude 8.5–9+ giants — the 2004 Indian Ocean and 2011 Japan events were both of this type.

By contrast, several common kinds of large earthquake make little or no tsunami:

• On-land quakes don't displace ocean water in the first place.
• Very deep quakes are too far below the seabed to move it much at the surface.
• Strike-slip quakes, where the ground slides horizontally past itself rather than thrusting up, shift water sideways rather than lifting it — so even a huge one may barely ripple the sea.

This is why magnitude alone is a poor predictor. A magnitude 7.8 strike-slip quake on land can be near-silent at sea, while a shallow undersea megathrust of the same number can be deadly. Location, depth and fault type are the deciding factors — and it's exactly those details, not just the big number, that tsunami warning centres race to assess in the first minutes.

Fast as a jet, then a wall at the coast

If a tsunami is generated, it crosses deep ocean at extraordinary speed — roughly 500–800 km/h, comparable to an airliner. Out there it's almost invisible: a wave only inches tall but tens of kilometres long, sliding under ships unnoticed.

Everything changes at the coast. As the wave reaches shallow water it slows dramatically and the energy stacks up, turning that low ocean swell into a towering, fast-moving surge. A coast far from the quake might get hours of warning as the wave races across the ocean. A coast right beside the rupture may get only minutes — which is why, near the sea, the earthquake itself is the warning. Strong or long shaking near a coast means move to high ground now, without waiting for an alert.

Reading the quake and the sea together

On the live map, this fusion plays out across two layers:

• Earthquakes shows recent events with their magnitude, depth and location. The details to read aren't just the magnitude — note whether it's offshore and shallow. That combination is the red flag.
• Sea State shows wave conditions. In the rare event of a confirmed tsunami, the threat is to coastlines, and the same ocean that ships read for storms becomes the thing coastal communities watch.

Most quakes you'll see on the map are land-based, deep, or modest, and the sea stays quiet. Learning to spot the dangerous combination — large, shallow, undersea — is what lets you understand, in the first moments, why some earthquakes trigger ocean-wide alarms and most don't.

This article explains the science of earthquake-generated tsunamis. For any real event, follow the guidance of your local official warning service.