New Zealand seafloor study offers insight into tsunami-causing quakes

Slow-motion earthquakes known as "slow slip events" offshore can lead to tsunami-generating regular earthquakes, according to research conducted in New Zealand out Friday.

The discovery was made in the world's first detailed investigation of seafloor movement at the Hikurangi subduction zone off the North Island's east coast, said a statement from the government's Institute of Geological and Nuclear Sciences (GNS Science).

Using a network of highly sensitive seafloor pressure recorders, an international team of scientists detected a slow-slip event that lasted two weeks in September 2014, resulting in 15 to 20 centimetres of movement.

If the movement, on the plate boundary between the North Island and the Pacific Plate, had occurred suddenly, it would have resulted in a magnitude 6.8 earthquake.

Slow-slip events were similar to earthquakes, but instead of releasing strain between two tectonic plates in seconds, they did it over days to weeks, creating quiet, centimeter-sized shifts in the landscape.

In a few cases, these small shifts had been associated with setting off destructive earthquakes, such as the magnitude 9 Tohoku-Oki earthquake that occurred off the coast of Japan in 2011 and generated a tsunami that caused the Fukushima Daiichi nuclear power plant disaster.

The slow-slip event studied in 2014 occurred in the same location as a magnitude 7.2 earthquake in 1947, which generated a large tsunami in the coastal area off the east of the North Island.

"Our results clearly show that shallow slow-slip event source areas are also capable of hosting seismic rupture and tsunami generation," study co-author Professor Yoshihiro Ito, of Kyoto University, said in the statement.

"This increases the need to continuously monitor shallow, offshore slow-slip events at subduction zones in New Zealand and elsewhere, using permanent monitoring networks similar to those that have already been established off the coast of Japan."

Earthquakes were unpredictable, but the link between slow-slip events and earthquakes could eventually help in forecasting the likelihood of damaging earthquakes and tsunamis, project leader Laura Wallace, of The University of Texas, said in the statement.

"To do that we will have to understand the links between slow-slip events and earthquakes much better than we currently do," said Wallace. Endit