April 20, 2026

expert reaction to earthquake and tsunami warning in Japan

Scientists comment on earthquake and tsunami warnings in Japan. 

Dr Ioannis Karmpadakis, Associate Professor in Coastal Engineering at Imperial College London, said:

• “What matters here is not whether the wave looks dramatic on camera, but what the water does when it reaches the coast.
• The shape of this coastline can turn a regional tsunami into a very local problem, with harbours, bays and river mouths all behaving differently.
• This situation is serious enough to respond immediately, but not on the scale of the 2011 Tōhoku disaster.

 Japan’s response

• Even waves that do not look dramatic can still be dangerous. A tsunami does not need to be towering to cause harm: the real danger is fast-moving water surging in and out, creating currents strong enough to knock people over, damage boats and push water up rivers and into harbours.
• Sanriku is one of the world’s classic tsunami-prone coastlines. Its saw-toothed shape, with many narrow bays and inlets, can funnel wave energy into particular places and make the effects worse locally.
• Reflected and refracted waves are part of the story. On a complicated coastline, tsunami waves can bounce around, bend and build up inside bays and harbours, so later surges can sometimes be worse than the first reading suggests.
• The warning map is not simply a map of distance from the earthquake. It is a map of how the sea is likely to behave along different parts of the coast, depending on seabed shape, coastline shape and the direction in which the tsunami energy is travelling.
• That means nearby places can face very different problems. One harbour may see dangerous currents, one river mouth may see water pushed inland, and one low-lying area may be more at risk of flooding.
• Ships leaving port is a sensible response. In deep water a tsunami may pass with little notice, but in shallow harbours it can turn into powerful surges and currents that strain moorings, move vessels around and cause collisions.
• Japan’s warning system is built for speed. The first alerts are designed to get people moving quickly, even before scientists have the full picture of exactly how much the seabed moved and how much water was displaced.
• The shape of the coast changes the kind of damage you get. Jagged coasts such as Sanriku can intensify waves and currents in bays, while flatter low-lying areas such as the Sendai Plain are more vulnerable to water spreading farther inland.

 Geological context

• An earthquake of about Mw 7.5 like this one is serious, but not globally exceptional. USGS says the long-term global average is about 16 major earthquakes a year: roughly 15 in the magnitude-7 range and 1 magnitude 8 or larger. Its 2000–2021 record shows 6 to 23 earthquakes per year in the 7.0–7.9 band, so an event of this size is rare for any one coastline but part of the normal global background of major seismicity.
• Japan is one of the least surprising places on Earth for an event like this. Statistics show that about 10% of the world’s earthquakes occur in and around Japan, and about 18.5%–20% of the world’s magnitude-6+ earthquakes, despite Japan having well under 1% of the world’s land area.

The 2011 Tōhoku event was Mw 9.0–9.1, the largest recorded in Japan since instrumental records began in 1900 and one of the largest worldwide. Each whole magnitude step corresponds to about 32 times more energy release, which means a Mw 9.0 releases about 180 times the energy of a Mw 7.5, and a Mw 9.1 about 256 times.”

Declared interests

Dr Ioannis Karmpadakis: “No conflict.”