A gigantic earthquake of magnitude 9.0 on the Richter scale occurred in the Japan Trench east of Honshu Island on Friday, March 11. It set off a tsunami with waves as high as 10 metres, leaving more than 17000 dead or missing. Hundreds of aftershocks, some of magnitude 6.0 or greater, are still to be felt.
At DHI, we have looked in some detail into what happened during one of the largest releases of energy in human history. Our hearts and thoughts are with all the people of Japan and all the families and friends affected by this disaster. We hope that through illustrating the physical processes we can help to understand how the disaster unfolded.
For now, we are able to show a detailed analysis of the wave propagation through the entire Pacific, all the way to the US Coast. Even there, some 8000 km away from the epicentre, the wave caused heavy destruction mainly in the harbours of Crescent City and Santa Cruz, California. A detailed model of the tsunami hitting the Japanese coast will be available within the next days.
How the wave forms and travels
Following a series of earthquakes, which in their own right were quite significant but turned out to be just preambles to the major shake, an incomprehensible 9.0-magnitude quake occurred at a depth of only 24km below the seafloor. In less than one minute, the Pacific plate slid 15 meters under the North American plate, which lies in between the Pacific and the Eurasian plate in that region. As a result, the North American plate and the overlying water was pushed upwards along a stretch of approximately 300 km– some 4 meters vertically into the sea at two hotspots and some 2 meters throughout the length of the fault. This caused a wall of water to form parallel to the coast. Such a wave starts travelling downhill –practically without feeling any friction from the seabed due to the very long wave length. That’s why tsunamis can travel at incredible speeds of up to 800 km per hour and propagate several thousands of kilometres. As the water depth destines the travelling speed of the wave, this speed varies depending on where the wave is headed. Our simulation clearly shows that the tsunami doesn’t spread uniformly in all directions.
No early warning
The tsunami wave generated off Japan was somewhat unusual: It did not have the typical negative wave on the side of the subducting plate, as this plate rather pushed forward than downward under the overlying North American plate. Therefore, when reaching the American coast it did not give the normal warning of the water receding first before the tsunami hits. On the Japan side there were no warnings either, and the tsunami wave travelled the 150 km to the coast in only 20 minutes at a speed of 450 km per hour.
Hitting land
As the tsunami approaches the coast, the shallower water causes the wave height to increase. Just like any other wave, it becomes shorter and higher, and breaks into a number of waves. It also turns with the angle of the coast. In bays and fjords, the energy is focussed - with devastating effects. One such area is around Sendai, where the focussing of the energy is very apparent, with the water reaching heights above 10 meters.
Once the wave swells above the tsunami defences and hits houses and other structures, it slows down and starts to loose energy. Nevertheless, it continues to travel inland, basically until it reaches higher ground. It is not only the depth of the water but also the sheer momentum at which it travels which creates the forces which normal buildings have not been built to withstand.
A close look at the water masses hitting the Japanese coastline is underway. Hopefully, such a detailed analysis of this event will help to minimise impacts in the future and can inspire in the reconstruction of this coastline.
Further validation of the Tsunami flooding impacts is ongoing, we will be updating this news bulletin shortly.