Japan earthquake: What caused Japan's massive tsunami?

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The earthquake which rocked Japan triggered tsunami alerts across the entire Pacific basin, including Russia, Mexico, Hawaii and the entire western US coast.

The area is so volatile, it is often referred to as the "ring of fire".

Measuring 8.9, the earthquake was 8,000 times more powerful than the one that devastated Christchurch in New Zealand in February.

David Shukman reports.

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