Nature's school room.

The US Transportable Array is a fleet of 400 high-end seismographs dotted around North America and the measurements of the Japanese earthquake recorded by these instruments has given scientists something to re-think about mega quakes. The magnitude-9 megathrust has been the subject of intense analysis especially from the Japanese who have an incredibly dense network of GPS and seismic coverage.

A subduction earthquake where one tectonic plate pushes underneath another rips in one or two directions along a fault line but this quake came in bursts of energy rips shown in the chart below.

Hiroo Kanamori of CalTec analysed the rupture zone which he said was split into two areas. One was a rupture zone along the Pacific Ocean's Japan Trench which he believes was responsible for the Tsunami and another rupture along the fault line, deeper and closer to the coast, caused most of the shaking.

The estimates of how far the tectonic plates slid past one another is almost 60 metres and if correct, is a massive shift unprecedented in the recorded history of earthquakes. What is more surprising is that the slip happened on a fault in the ocean crust that is about 140 million years old and megathrusts quakes are more usual on young, hot, swiftly subducting plates.

Now scientists are realising that previous models of which type earthquakes would hit where are no longer set in lava (joke, laugh please). A magnitude 9 is just waiting to happen anywhere on a subduction zone.