Oceanic crust

More than one ocean motion determines tsunami size

2011 tsunami visualized
OCEAN MOTION The 2011 tsunami that devastated Japan (forecast shown) was fueled by both the horizontal and vertical motion of the seafloor, new research suggests. Darker colors represent higher waves. Black triangles mark tsunami-measuring buoys.

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…

Is Zealandia a continent?

New Zealand
New Zealand

Lurking beneath New Zealand is a long-hidden continent, geologists now propose. They call it Zealandia. Don’t expect it to soon end up on a map on your classroom wall, though. Nobody is in charge of officially designating a new continent. Scientists will have to judge for themselves if Zealandia should be added to the ranks of continents.

A team of geologists pitched the scientific case for judging this a new continent in the March/April issue of GSA Today. Zealandia is a continuous expanse of continental crust. It covers some 4.9 million square kilometers (1.9 million square miles). That’s about the size of the Indian subcontinent. But it would be the smallest of the world’s continents. And unlike the others, around 94 percent of Zealandia hides beneath the ocean. Only New Zealand, New Caledonia and a few small islands peek above the waves over it.

“If we could pull the plug on the world’s oceans, it would be quite clear that Zealandia stands out,” says study coauthor Nick Mortimer. He is a geologist at GNS Science in Dunedin, New Zealand. Zealandia rises about 3,000 meters (9,800 feet) above the surrounding ocean crust, he notes. “If it wasn’t for the ocean level,” he says, “long ago we’d have recognized Zealandia for what it was — a continent.”

Story continues below map

continents map
A landmass called Zealandia (gray region) deserves to join the ranks of continents, some geologists now propose. Only 4 percent of Zealandia rises above sea level (dark gray), including New Zealand. But swaths of other continents also are submerged along their margins (light-shaded regions).

This landmass, directly east of Australia, will face an uphill battle for continent status. New planets and slices of geologic time have international panels that can officially name them. But there is no such group to officially validate new continents. The current number of continents is already vague. Most everyone agrees on five of them: Africa, Antarctica, Australia and North and South America. Some people, however, combine the last two — Europe and Asia — into one huge Eurasia. There’s no formal way to add Zealandia to this mix. Proponents will just have to start using the term and hope it catches on, Mortimer says.

This odd path forward stems from the simple fact that nobody expected another continent would ever need to be added, says Keith Klepeis. He is a structural geologist…