Dr David Rothery, Open University Volcano Dynamics Group
This morning's Japanese earthquake (05:46 GMT) measured 8.9 on the Richter scale. That is very powerful, and in an average year there is only one quake more powerful than 8.0 anywhere in the globe. At its source, it was over a thousand times more energetic than the magnitude 6.3 quake that struck Christchurch on 22 February. This one occurred about 25 km below the seabed, and the displacement of seawater caused a series of tsunami waves capable of causing far more damage than the on-land ground shaking.
Parts of the eastern coast of Japan have already been inundated, but there is a tsunami warning in force across most of the Pacific basin. For example, the first waves are expected to reach Hawaii seven or eight hours after the earthquake.
This earthquake was a consequence of the floor of the Pacific Ocean being dragged under Japan as a result of plate tectonic movements. It was preceded by a nearby magnitude 7.2 quake on 9 March and there was a magnitude 7.1 aftershock at 06.25 GMT.
The 2004 Boxing Day tsunami in the Indian Ocean was caused by a magnitude 9.1 earthquake where the floor of the Indian Ocean is dragged below Sumatra.
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