New light on how tsunamis form

What causes a tsunami? We thought we knew. The energy from a massive uplift of the seafloor caused by subduction – where one tectonic plate slides under another – during an earthquake transmits the force to the water above. Right? Well maybe not.

That understanding that the upwards movement of the seafloor was the cause of tsunamis was the result of 1970s wave tank experiments. The theory was reinforced when Japanese scientists later simulated horizontal seafloor displacements in a wave tank and found negligible energy transfers.

But that received wisdom has been turned on its head thanks to work by NASA oceanographer Tony Song. His analysis of the 2004 Sumatra earthquake in the Indian Ocean suggest that the vertical uplift of the seafloor did not produce enough energy to create a tsunami that powerful. 

The tsunami’s total energy can only be accounted for by adding in horizontal movement as well, he found.

Beginning in 2007, Song used data from the NASA/Centre National d’Etudes Spatiales (CNES) Jason satellite, the US Navy’s Geosat Follow-on and the European Space Agency’s Environmental Satellite.

Satellite data of the 2004 quake from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (GRACE) mission, also backed up his theory.

“I had all this evidence that contradicted the conventional theory, but I needed more proof,” Song said.

Going back to the earlier Japanese research, Song found the wave tank experiments of the 1980s into horizontal movement were flawed. The tanks did not accurately reflect two key variables that determine the amount of kinetic energy created by the movement – the movement proportional to the depth of the ocean and the speed of the seafloor’s movement.

The wave tanks used were too shallow and the floor moved too slowly to accurately model the phenomenon.

So Song and researchers from Oregon State University set about rectifying the earlier experiments, building tanks that were deemed and a floor moved by a piston-powered mechanism that worked faster.

Results of the new experiments showed that horizontal seafloor displacement contributed more than half the energy that generated the 2004 and 2011 tsunamis.

The study proves a pint but also has practical application for tsunami detection using GPS technology through the Global Differential Global Positioning System (GDGPS). 

This highly accurate real-time GPS processing system measures seafloor movement during an earthquake to calculate the size and direction of a tsunami.

“By identifying the important role of the horizontal motion of the seafloor, our GPS approach directly estimates the energy transferred by an earthquake to the ocean,” Song said. 

“Our goal is to detect a tsunami’s size before it even forms, for early warnings.”

The study was published in Journal of Geophysical Research – Oceans.