Scientists think they understand why the earthquake on the 11th of March off the coast of the island of Honshu, Japan, caused such a large and devastating tsunami. The high score on the Richter scale (9.0) is not a sufficient explanation in itself.

The fault line east of Honshu is a so called ‘subduction zone’, where the eastern part of the ocean floor slowly slides westwards, underneath the western tectonic plate, and into the depths of the Earth’s mantle.

This is not a constant motion: the tension between the two plates needs to first build up to overcome the friction forces, before substantial relative movement is possible.

‘Normal earthquake’

During this sliding motion the overlying plate, including the Japanese mainland, starts to shake. And that is indeed what happened during the first 40 seconds of the earthquake on March 11, says geophysicist Greg Baroza of Stanford University.

Upward rebound

After that something more remarkable happened. Instead of just shaking, the overlying plate started to move itself – though in the opposite direction, eastwards, away from the epicenter – and up towards the surface.

This ‘counter shock’ lasted for 30 to 35 seconds. The associated American and Japanese geoscientists explain in their publication in Science that this movement started at a depth of 32 kilometers and kept accelerating as it neared the bottom of the ocean.

Sudden bump at sea floor

A large amount of loose sea floor sediments were scraped together, quickly forming a hill at the bottom of the ocean that pushed an enormous amount of sea water upwards, creating the wave that led to the meters-high tsunami.

© Rolf Schuttenhelm | www.bitsofscience.org