How the great Tohoku-Sendai earthquake and tsunami happened

English: Sendai Rinkai Railway locomotive(SD55...

Railway locomotive thrown aside by the 11 March 2011 Tsunami in Japan. (credit: Wikipedia)

The great Tohoku earthquake (moment magnitude 9.0) of 11 March 2011 beneath the Pacific Ocean off the largest Japanese island of Honshu resulted in the devastating tsunami that tore many kilometres inland along its northern coast line and affected the entire Pacific Basin (see NOAA animation of the tsunami’s propagation) . The area and indeed Japan itself have yet to recover from the devastation almost 3 years later. Over 18 thousand people died, witnessed by hundreds of millions of television viewers. The Fukashima Daiichi nuclear reactor had a catastrophic meltdown and release of radioactive materials displacing, along with the urban destruction by the tsunami, a third of a million people, many of whom are yet to be properly housed.

The seismic trigger happened at a plate boundary where lithosphere of the Western Pacific is being subducted beneath Japan. Subduction zone seismicity extends from shallow depths to as deep as 700 km beneath the surface. The destructive nature of the Tohoku eathquake stemmed from its occurrence at a shallow depth (~20-30 km) that allowed the motion to shove crustal material eastwards, up and over the sea floor to cause the sea floor to bulge upwards by tens of metres in a matter of seconds. It was that surface-breaking megathrust that displaced Pacific Ocean water and launched the huge tsunami waves. Geophysicists were caught by surprise as regards the magnitude of the event, having long considered that part of the Pacific ‘ring of fire’ to be incapable of generating seismic energies above a magnitude of 8.0; 32 times less energetic than the magnitude 9.0 that was reached in reality. The area to watch was believed to be the southwestern coastline of Japan, affected by subduction beneath the Sagami and Suguma Troughs. The reason for this attempt at anticipation in what is one of the world’s most risky places for seismicity is that theory suggested that subduction slip was greatest at depth and becomes smaller at shallower levels.

Clearly, a major scientific effort had to be undertaken to explain such a disastrous misconception. Part of this involved drilling into the seabed above the 11 March 2011 epicentre. The extracted rock cores revealed a major surprise (Chester, F.M. and 14 others. 2013. Structure and composition of the plate-boundary slip for the 2011 Tohoku-Oki earthquake. Science, v. 342, p. 1208-1211): the fault zone was a layer of clayey rock less than 5 m thick with a rupture zone for the Tohoku earthquake estimated at only a few centimetres across. Experiments revealed that hardly any heat had been generated by such a huge earthquake (Fulton, P.M. and 9 others 2013. Low coseismic friction on the Tohoku-Oki Fault determined from temperature measurements. Science, v. 342, p. 1214-1217). Friction had been extremely low, probably because the clay was so impermeable that water pressure in it was able to build up and not diffuse away (Ujie, K. and 9 others 2013. Low coseismic shear stress on the Tohoku-Oki Megathrust determined from laboratory experiments. Science, v. 342, p. 1211-12145). The thrust fault was lubricated, but fortunately one that was localised: unlike the strike-slip fault that drove the Indian Ocean tsunamis of 2005 which was able to propagate for over 1000 km.

While there is cause for some satisfaction among seismologists for a technical explanation, how the findings can be applied to better prediction of tsunami-prone subduction zones is not very clear.  It does seem that the Tohoku-Oki Fault has developed, probably over millions of years, in particularly clay-rich sea-floor sediments. Such a phenomenal amount of slippage would be less likely in coarser shallow sediments that would probably generate much more friction. Putting the findings into practice will involve greater investment in and speeding up oceanographic studies of submarine trench systems.

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