Home / News and Events / Media Releases / Earthquake instruments to be put on the seafloor in poverty bay - 12/05/2014

Earthquake instruments to be put on the seafloor in poverty bay - 12/05/2014

A group of international scientists will place instruments on the seafloor near Poverty Bay during the next 10 days to measure earthquake activity and movement of the seafloor where the Pacific and Australian tectonic plates meet.

The 32 instruments, belonging to the United States and Japan, will remain on the seafloor for one year recording earthquakes and any upwards or downwards movement of the seafloor.

The deployments, which are part of a joint project involving New Zealand, Japan and the United States, will take place between 25km and 100km east of Gisborne in water depths ranging from 100m to 4000m.

NIWA’s deepwater research ship Tangaroa is being used for the operation, which will involve lowering the instruments over the side of the ship at predetermined locations.

The project is designed to give valuable insights into the earthquake and tsunami potential of the Hikurangi subduction zone, which lies to the east of Gisborne several kilometres below the seafloor.

Dr Spahr Webb

Voyage co-leader Dr Spahr Webb of Columbia University in New York loads some of the ocean bottom seismometers onto the Tangaroa before setting off for Poverty Bay where they will be placed on the seafloor for 12 months. Photo - John Callan, GNS Science

This is where the Pacific plate, to the east of the North Island, is being thrust under or “subducted” beneath the Australian plate.

Offshore Poverty Bay is notable for a phenomenon known as slow-slip events or “silent earthquakes”, which are a focus of this project.

Slow-slip events are similar to earthquakes in that they involve more rapid movement than normal across a fault. However, they occur more slowly, in a matter of weeks to months, compared to earthquakes which occur in a matter of seconds.

“Highly sensitive pressure recorders on the instruments will be able to detect vertical movements as small as 0.5cm of the seafloor in slow-slip events, allowing us to understand the distribution of slow-slip offshore Poverty Bay for the first time,” said one of the project leaders, Laura Wallace of the University of Texas.

“As well as vertical movements, we expect that the instruments will record many hundreds of small earthquakes that cannot be accurately located with land-based instruments,” Dr Wallace said.

“The data will help to paint a picture of the inner workings of the plate boundary under Poverty Bay.”

In particular, the instruments will provide accurate earthquake depths which is important for revealing the three-dimensional distribution of earthquakes within the Earth’s crust.

The instruments will store their data on-board and scientists will download and analyse it when they are retrieved from the seafloor in 2015, using a United States research ship.

Voyage co-leader Dr Stuart Henrys of GNS Science with some of the 32 deep-sea seismic instruments that will be placed on the seafloor off Poverty Bay during the next 10 days. Photo - John Callan, GNS Science

Voyage co-leader Dr Stuart Henrys of GNS Science with some of the 32 deep-sea seismic instruments that will be placed on the seafloor off Poverty Bay during the next 10 days. Photo - John Callan, GNS Science

This project is the largest-ever deployment of seafloor instruments specifically targeted at the study of silent earthquakes.

Occurring at roughly 18 month intervals in the Poverty Bay area, slow-slip earthquakes involve large parts of the region moving eastward by up to 2cm over one or two weeks, as detected by GPS instruments that are part of the GeoNet national network of instruments.

“The only reason we know that slow-slip events occur in New Zealand is because of the large network of permanent GPS instruments that are operated as part of the EQC-funded GeoNet project,” Dr Wallace said.

The last such events occurred in July and October 2013. The July event occurred east of Tokomaru Bay and the October event was just offshore between Mahia and Gisborne.

No-one feels or hears anything during slow-slip events, hence the name “silent earthquakes”, but if the land movement occurred in seconds rather than weeks, like a normal earthquake, it would be equivalent to a magnitude 6.0 to 7.0 jolt.

Poverty Bay is one of about a dozen areas worldwide where these events occur regularly. However, this region is unique in that they occur at depths ranging from 5km to 15km under the seafloor.

In most other places in the world they occur 20km to 40km under the surface. Scientists are not sure exactly what causes these events, and why they occur with such regularity. The shallow depth of the events makes Poverty Bay a very attractive area for scientists to investigate this phenomenon.

GNS Science technician Randall McDonnell prepares to load ocean bottom seismometers onto the Tangaroa before setting off for Poverty Bay where they will be deployed on the seafloor. Photo - Margaret Low, GNS Science

GNS Science technician Randall McDonnell prepares to load ocean bottom seismometers onto the Tangaroa before setting off for Poverty Bay where they will be deployed on the seafloor. Photo - Margaret Low, GNS Science

Project member and seismologist at GNS Science, Bill Fry, said the study of slow-slip earthquakes immediately east of the North Island would lead to a better appreciation of the earthquake and tsunami potential of this undersea fault system.

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

“Recent examples are the magnitude 9.0 Tohoku quake in Japan in April 2011 and the magnitude 9.1 Sumatran quake in 2004.”

Seismologists believe a megathrust quake of magnitude 9.0 is possible on the Hikurangi subduction zone.

“An earthquake of this size would produce damaging shaking throughout much of New Zealand plus a tsunami that would affect most of the country and potentially other parts of the Pacific as well,” Dr Fry said.

Poverty Bay deployment

SUCCESSFUL POVERTY BAY DEPLOYMENT, 22 MAY 2014 - More news.

Q&A

Why is this project important and why do it in Poverty Bay?
The region offshore Gisborne and Hawkes Bay is of great interest to Earth scientists because of ‘slow-slip events’ or ‘silent earthquakes’ that have been documented there since 2002. They have probably been occurring there for many thousands of years. Slow-slip or silent earthquakes are a recently discovered form of fault behaviour, where movement between the tectonic plates happens across the subduction plate boundary slowly over a period of weeks to months, rather than suddenly in a large earthquake. The movement of the plates creates a lot of natural earthquake activity and this has been widely known for a long time. Understanding slow-slip events will almost certainly help scientists to understand their faster and more damaging cousins. Importantly, a major slow-slip event occurs off Gisborne roughly every two years.

Our knowledge of where on the plate interface these slow-slip events rupture has so far come from onshore GPS instruments. Since most of the slow-slip events occur offshore, we are not able to precisely measure which parts of the plate are slipping. To get a better picture of these events and associated small earthquakes that accompany them, we are deploying just over 30 ocean bottom seismographs and seafloor pressure sensors.

Describe the instruments being deployed off Gisborne:
There is a mix of Ocean Bottom Seismographs (OBS) and Seafloor Pressure Sensors. OBSs sit firmly on the seafloor and accurately measure earthquakes beneath the seafloor.

They are housed in either metal or glass cylinders capable of withstanding pressures at depths up to 3.5km. They have anchors that keep them grounded and they are able to store up to a year’s worth of data on board. Their data gives information on the structure of the Earth’s crust down to about 15km below the seafloor. When the time comes to retrieve them, scientists send an acoustic signal and they detach from their anchor and float to the surface where they are picked up by a waiting ship.

Pressure sensors measure any very small up and down movement of the seafloor. All the instruments will remain recording on the seafloor for a year. In that time, scientists anticipate at least one slow-slip event will occur and the instruments will record the various phenomena associated with it.

What sort of outcome would be seen as a success?
For this to be achieved, the first thing is that all the instruments will need to work flawlessly for 12 months recording the information they are designed to record. Secondly, in 2015 scientists will need retrieve the instruments and download their stored data and obtain meaningful results from it. If a large slow-slip event occurs offshore from Gisborne during the deployment, the seafloor instruments, together with the onshore data, will enable scientists to probe the event in detail. At present, it is unclear if the plate boundary slips all the way to the seafloor or is just confined to depths greater than 5km.

Scientists also want to know the distribution and timing of any associated small earthquakes.

Do they occur before or after a slow-slip? Do they occur around the slip patch in one burst or many bursts? This will reveal how the stresses are moving around inside the Earth’s crust. This information is important because it might tell us how the plate will behave in a very large earthquake, or if it has the potential to create tsunamis. Because the plate boundary zones, such as the one east of Poverty Bay, are the sites of the most damaging and destructive earthquakes in the world, scientists use multiple investigation techniques to advance their understanding of these areas.

The region offshore Gisborne is an excellent ‘laboratory’ in which to learn new information on the way the Earth works. This deployment is part of a multi-year package of scientific investigations in the Poverty Bay area that will help significantly in advancing earthquake science.