we touched this same spot with our hands, our feet, our gaze and our dreams

Thursday, April 07, 2011

The anatomy of a megathrust earthquake

It was the biggest quake to rock Japan in recorded history. On 11 March 2011, just 130 km off the coast of Sendai, on the main island of Honshu, a section of the massive submarine fault separating the enormous Pacific plate from northern Japan failed.

In a few minutes, over 500 km of the fault catastrophically ruptured, and rocks on either side were thrown over 20 metres along the fault.

The island of Honshu was wrenched 2.5 m towards the east, and the Earth's tilt axis moved by 16 cm in response to this enormous redistribution of mass. The seafloor upheaval shunted a column of water over 6 km deep upwards - a massive disturbance which culminated in a 10 m tsunami swamping the Japanese coastline.

At magnitude 9, the quake was propelled into another class of monster - a megaquake. The scale used to measure earthquake magnitude, whilst useful, does not do justice to the energy involved in the biggest of the big.

The moment magnitude scale is logarithmic - meaning the energy released by a magnitude 9 is around 32 times greater than a magnitude 8, and almost 1,000 times larger than a magnitude 7.


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