Successful Poverty Bay deployment - 22 may 2014

The ten-day voyage was successful with all the instruments being deployed on the ocean floor off the Poverty Bay coast with a day to spare. The crew also retrieved instruments that were deployed last year and were able to take an initial look at the data that had been recorded on them.

Project co-leader Stuart Henrys, of GNS Science, said the recovered equipment had revealed some evidence of ‘slow-slip’ earthquakes that had occurred during the past 12 months.

"There are some promising signals from the data, which clearly bodes well for the deployment we have just completed off Poverty Bay,” Dr Henrys said.

Preliminary analysis suggests the recovered instruments may have captured the offshore signal of two slow-slip events along the east coast of the North Island in the past 12 months.

The events were also recorded by land-based equipment. The combination of the land and seafloor observations will enable scientists to better define the location of slow-slip events.

"The instruments that we retrieved are a new type of geophysical instrument from Tohoku University, Japan, that record pressure changes on the seafloor. They detected small vertical movement of the sea bed, accurate down to a centimetre.

“Continuous GPS measurements onshore provide precise horizontal and vertical measurements of the land surface, but GPS signals will not travel beneath the ocean. Only recently has technology advanced to the point where the detection of small vertical motion is now feasible. The Poverty Bay deployment will be a significant test of this technology,” Dr Henrys said.

The equipment deployed last week is expected to record "thousands" of small magnitude tremors that might not be detected by more than a few of the GeoNet national network of on-land instruments.

Scientists hope the data will help to establish if periodic episodes of high earthquake activity are connected to slow-slip events.

This year’s deployment will be followed by smaller deployments off Poverty Bay next year and beyond.

“We hope we can secure the funding to undertake these rolling deployments so as to learn as much as we can about the different types of earthquake behaviour off the Gisborne coast.”

Subduction zones, such as the one offshore from the North Island, are responsible for generating the world’s largest earthquakes, sometimes called megathrust quakes.

Dr Henrys said Poverty Bay and the area to the east of Wellington and Wairarapa gave contrasting views of the plate margin, in terms of earthquake potential.

“The Gisborne area is particularly interesting because it has these slow-slip events at shallow depths. In contrast, the area to the south, offshore Wellington, doesn’t exhibit shallow slow-slip behaviour, but does experience these type events at greater depths beneath the Kapiti coast. But directly underneath Wellington, the two plates are locked.”

Crew on the Tangaroa assemble ocean bottom seismographs on the deck of the ship prior to deploying them off the Poverty Bay coast.  The instruments, worth roughly $100,000 each, are capable of storing 12 months' of data on board and can operate comfortably at depths up to 3km.  Photo Dr Stuart Henrys, GNS Science

Crew on the Tangaroa assemble ocean bottom seismographs on the deck of the ship prior to deploying them off the Poverty Bay coast. The instruments, worth roughly $100,000 each, are capable of storing 12 months' of data on board and can operate comfortably at depths up to 3km. Photo Dr Stuart Henrys, GNS Science