Where the Pacific plate collides with the Indo-Australian plate, it is forced downwards into the trench, a subduction zone, and the volcanoes are carried with it.

BBC: Undersea mountains march into the abyss

The Australian plate borders the Pacific plate to the East (the Samoan quake occurred along this boundary), while on the Western side, it borders the Eurasian plate (where the Indonesia quake occurred).

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But on that fateful Friday, when the North American plate slid over the Pacific plate along a subduction zone running 130km (80 miles) off the Pacific coast of northern Japan, the shock leapt from the first segment to a second and on to a third, extending the fault zone some 400km and increasing its intensity more than 30-fold.

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The volcanoes are strung across several thousand kilometres of ocean floor and are moving westward on the Pacific tectonic plate at up to 6cm per year.

BBC: Undersea mountains march into the abyss

The effects of the 2011 tsunami in Japan were felt across the Pacific Ocean to earthquake-prone California, which shares the same tectonic plate.

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