More than one ocean motion determines tsunami size

Earthquake-powered shifts along the seafloor that push water forward, not just up, could help supersize tsunamis.

By combining laboratory experiments, computer simulations and real-world observations, researchers discovered that the horizontal movement of sloped seafloor during an underwater earthquake can give tsunamis a critical boost. Scientists previously assumed that vertical movement alone contributed most of a tsunami’s energy.

More than half of the energy for the unexpectedly large tsunami that devastated Japan in 2011 (SN Online: 6/16/11) originated from the horizontal movement of the seafloor, the researchers estimate. Accounting for this lateral motion could explain why some earthquakes generate large tsunamis while others don’t, the researchers report in a paper to be published in the Journal of Geophysical Research: Oceans.

“For the last 30 years, we’ve been moving in the wrong direction to do a good job predicting tsunamis,” says study coauthor Tony Song, an oceanographer at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. “This new theory will lead to a better predictive approach than we have now.”

The largest tsunamis form following earthquakes that occur along tectonic boundaries where an oceanic plate sinks below a continental plate. That movement isn’t always smooth; sections of the two plates can stick together. As the bottom oceanic plate sinks, it bends the top continental plate downward like a weighed-down diving board. Eventually, the pent-up stress becomes too much and the plates abruptly unstick, causing the overlying plate to snap upward and triggering an earthquake. That upward movement lifts the seafloor, displacing huge volumes of water that pile up on the sea surface and spread outward as a tsunami.

These deep-sea earthquakes shift the seafloor sideways, too. The earthquake off the coast of Japan in 2011, for instance, not only lifted the ocean floor three to five meters; it also caused up to 58 meters of horizontal movement. Such lateral motion, however big, is mostly ignored in tsunami science, largely because of a 1982 laboratory study that found no connection between horizontal ground motion and wave height. The experiment used in that study, Song argues, wasn’t a properly sized-down model of the dimensions of the seafloor and overlying ocean. If lateral motion takes place on a sloped segment of the seafloor, he thought, then the shift can push large volumes of water sideways and add momentum to the budding tsunami.

Using two wave-making machines at Oregon State University in Corvallis, Song and colleagues revisited the decades-old experiment. Oarlike paddles pushed water upward and outward in some tests and just upward in others. Adding horizontal motion caused higher waves than vertical motion alone, the researchers found.

By combining the experimental results with a new tsunami computer simulation that incorporates lateral movement, the researchers could account for the unusual size of the 2004 Indian Ocean tsunami. That tsunami, one of the worst natural disasters on record, was bigger than uplift alone can explain.

Using GPS sensors to measure the horizontal movement of the seafloor during an earthquake will enable more accurate tsunami forecasts before the wave is spotted by ocean buoys, Song proposes.

The new work makes a convincing case that horizontal motion contributes to tsunami generation, says Eddie Bernard, a tsunami scientist emeritus at the National Oceanic and Atmospheric Administration’s Center for Tsunami Research in Seattle. But just how much that movement contributes to a tsunami’s overall height is unclear. It could be much less than Song and colleagues predict, he says.

Other seafloor events that can follow a large earthquake — such as huge numbers of water-displacing landslides — could also boost a tsunami’s size. Until all of the factors are known, Bernard says, tsunami forecasters will probably be best off doing what they do now: waiting for a tsunami to form after an earthquake before predicting the wave’s size and trajectory.