Home / News and Events / Media Releases / Sub-seafloor probe to give clearer picture of offshore quakes - 06/09/2016

Sub-seafloor probe to give clearer picture of offshore quakes - 06/09/2016

Scientists who struggled to unravel the complexities of last week's magnitude 7.1 quake northeast of East Cape say a brace of undersea instruments to be installed east of Gisborne in 2018 should ultimately lead to more accurate information on undersea quakes east of the North Island.

Thanks to the 26-nation International Ocean Discovery Program (IODP), New Zealand will become one of only a few countries in the world to have a sub-seafloor 'earthquake observatory' off its coast.

During a two-month voyage in 2018, the US-based research ship JOIDES Resolution will drill three holes up to 1.5km deep in the seafloor in different parts of the overlying Australian tectonic plate about 40km east of Gisborne.

The project is one of five back-to-back IODP expeditions the research ship will undertake in New Zealand waters starting in late 2017. Each project will last about two months and the ship will be in New Zealand waters for nearly a year.

Into one of the boreholes east of Gisborne, the ship will insert a range of instruments to make continuous physical and chemical measurements inside the plate boundary zone. The instruments will be in place for at least the next decade, and will detect any changes due to earthquakes and other types of tectonic events in the region.

The ship will also collect samples of sediment and rock from the drill holes for scientists to analyse.

Sea-seafloor probe

A key focus for scientists involved in the project will be to learn how the temperatures and pressures at depth, and the types of rocks being subducted, might influence the occurrence of earthquakes and slow-slip events at the subduction zone - where the Pacific plate is being forced under the Australian plate.

Geophysicist Laura Wallace of GNS Science, who is leading the offshore Gisborne expedition, said the project's findings will almost certainly have global significance.

"It has the potential to significantly boost the understanding of the mechanics of subduction zone faults and the earthquakes that occur on them," Dr Wallace said.

The project's main aim is to improve the understanding of slow-slip or silent earthquakes which are feature of the subduction zone east of the North Island.

But Dr Wallace said it would have multiple benefits in understanding a range of plate boundary phenomena. The project involves at least 50 scientists from several countries.

For the past three years, there has been a 'rolling deployment' of seafloor sensors off the Gisborne coast to monitor slow-slip activity and earthquakes on the Hikurangi subduction zone east of the North Island.

On loan from Japanese and US institutions, the instruments sit on the seafloor for 12 months recording offshore quake activity with much greater precision than land-based instruments because of their proximity to the action.

They store data on board while remaining on the seafloor, and after a year they are retrieved by a ship and the data is recovered. The ship then deploys fresh instruments at the same location.

"These seafloor instruments have given us tremendous insights into the workings of this complex plate boundary fault zone, but the information is always retrospective."

Dr Wallace said after last week's earthquake northeast of East Cape, there was a case to be made for data from the seafloor instruments to be sent onshore in real-time using fibre optic cable.

"A continuous quake monitoring capability east of the North Island would be enormously helpful in speeding up the analysis and understanding of complex offshore earthquakes that might potentially have generated a tsunami."

The distances between offshore quake epicentres and land-based seismometers meant it was difficult for New Zealand's current national network, operated by GeoNet, to quickly resolve the depth, location and magnitude of offshore earthquakes, Dr Wallace said.

Knowing this kind of information with confidence is crucial to issuing timely tsunami warnings.

Background

‘Silent earthquakes’, also known as slow-slip events, are similar to an earthquake in that they involve more rapid than normal movement between two pieces of the Earth’s crust along a fault line. However, in a normal earthquake the slip occurs in a matter of seconds, suddenly releasing seismic energy, whereas slip in a silent earthquake can take weeks or even months to occur.

The discovery of silent earthquakes has been one of the most important findings in seismology in the past two decades. Scientists have proposed numerous theories to explain why silent earthquakes occur, but testing the theories is challenging as these events typically occur tens of kilometres underground.

Silent earthquakes offthe coast ofGisborne have attracted significant attention from international scientists due to their close proximity to the Earth’s surface – typically about 5km beneath the seafloor. This means they are accessible to scientific drilling.This contrasts with many other parts of the world where this phenomenon occurs tens of kilometers below the Earth's surface.

The International Ocean Discovery Program is an international marine research collaboration that is funded by 26 nations dedicated to advancing scientific understanding of the Earth by sampling, instrumenting and monitoring subseafloor environments using specialised ocean drilling ships. New Zealand has been a member of IODP since 2008, as part of the Australia New Zealand IODP Consortium (ANZIC).