Seafloor sensors off New Zealand to yield secrets of "silent quakes"

An international team of scientists will this month begin retrieving the world's largest- ever deployment of seafloor quake measuring instruments off New Zealand as part of a study to understand earthquake and tsunami risks below the sea floor.

Scientists from the United States, Japan and New Zealand will retrieve instruments from the seafloor near Poverty Bay, off the east of the North Island, over the next two weeks, the government' s Institute of Geological and Nuclear Sciences (GNS Science) said Wednesday.

Data from the instruments would be analyzed to measure slow- slip events or "silent earthquakes" and movements of the seafloor where the Pacific Plate is being thrust under the eastern North Island.

Slow-slip events are similar to earthquakes, but occur more slowly, over weeks or months, compared to regular earthquakes that occur in a matter of seconds.

The project started in May last year when the team deployed 35 instruments that remained on the seafloor recording earthquakes and any upwards or downwards movement of the seafloor.

Scientists said they were anticipating exciting results, as a large slow-slip event occurred beneath Poverty Bay in late September-early October last year, directly beneath the seafloor network.

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

"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," Wallace said.

Occurring at one to two-year intervals, slow-slip events in Poverty Bay involved large areas moving eastward by up to two to three cm over one or two weeks - equivalent to a magnitude six or seven jolt if it occurred in seconds like a normal quake.

Poverty Bay was one of about a dozen areas worldwide where silent earthquakes occurred regularly, but the region was unique in that they occurred at "shallow" depths ranging from five km to 15 km under the seafloor. Endi