Quake Instruments to Be Retrieved After 12 Months

MEDIA RELEASE from GNS Science
17 JUNE 2015

Quake Instruments to Be Retrieved After 12 Months on the Seafloor

Scientists from the United States, Japan, and New Zealand will retrieve instruments from the seafloor near Poverty Bay over the next two weeks as part of a project to measure earthquake activity and movement of the seafloor where the Pacific Plate is being thrust under or 'subducted' beneath the eastern North Island.

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 than normal movement across a fault.

The main difference is that slow-slip events occur more slowly, over weeks or months, compared to regular earthquakes which occur in a matter of seconds.

The 10-day voyage, which leaves Napier on June 20, is funded by the United States National Science Foundation and will be conducted aboard the US research ship, R/V Roger Revelle, operated by Scripps Institution of Oceanography.

The project constitutes the world’s largest-ever deployment of seafloor instruments specifically designed to study slow-slip events. It started last May when the international team deployed 35 instruments on the seafloor off the
Gisborne coast using NIWA's research ship Tangaroa.

They have remained on the seafloor for the past year recording earthquakes and any upwards or downwards movement of the seafloor.

Called ocean-bottom seismometers, the instruments range in size from that of a large suitcase to the size of a washing machine, and weigh between 130kg and 200kg each. Retrieval involves sending an acoustic signal so each instrument floats back to the sea surface where it will be recovered by the ship.

Once the instruments are on board the Roger Revelle, the scientists will download data that has been recorded over the past year. It is likely to take months to complete the data analysis that will reveal the behaviour of the plate boundary off the Gisborne coast.

If the data recovery is successful, scientists anticipate some exciting results as a large slow-slip event occurred beneath Poverty Bay in late September-early October of last year, directly beneath the seafloor network. The slip event was detected by onshore GPS instruments in the EQC-funded GeoNet network, which is operated by GNS Science.

"Highly sensitive pressure recorders on the undersea instruments will be able to detect vertical movements of the seafloor as small as 0.5cm during the September-October 2014 slow-slip event, and will reveal the extent of offshore slow-slip for the first time ever,” said project leader Laura Wallace of the University of Texas.

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

Project member and seismologist at GNS Science, Dr Stephen Bannister said: “We also expect the seismological data collected to shed light on the location and origin of the swarm of earthquakes occurring off the coast of Ruatoria
that have been felt in recent months, as well as earthquakes that occurred during the 2014 slow-slip event.”

Occurring at one to two year intervals, slow-slip events involve large areas of Poverty Bay moving eastward by up to 2-3cm over one or two weeks, as detected by GPS instruments.

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

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 to 7 jolt.

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

The unusually shallow depth of the events beneath Poverty Bay makes it a very attractive area for international scientists to investigate this phenomenon.

Yoshihiro Ito and Kimihiro Mochizuki are two of the Japanese scientists involved in the project, and they say that investigating New Zealand’s Hikurangi subduction zone will help them to better understand slow-slip
events and earthquakes at their own subduction zones off the coast of Japan.

A collaborator and seismologist on the project, Dr Bill Fry of GNS Science, expects the new data will also lead to a better appreciation of the earthquake and tsunami potential of this undersea fault system.

The 35 instruments being retrieved belong to Columbia University and the University of Texas in the US, and to the University of Tokyo and Tohoku University in Japan.

Scientists from seven institutions in the US and Japan are involved in the project including: The University of Texas, Columbia University, the University of California-Santa Cruz, the University of Colorado, University of Tokyo, Kyoto University, and Tohoku University. Several scientists from GNS Science are also involved, and two of the voyage participants are from Victoria University of Wellington.

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