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Scientists survey Hikurangi Margin off Poverty Bay - 04/10/2010

Marine geologists from GNS Science and NIWA will spend the next 16 days on NIWA’s deepwater research ship Tangaroa off the Poverty Bay coast collecting information on the Hikurangi Margin.

This is a major feature under the seafloor where the Pacific Plate thrusts beneath the Australian Plate at a rate of 4cm to 5cm-a-year. The North Island stands on the Australian Plate.

Subduction margins around the world, similar to the one off the Poverty Bay coast, can generate megathrust earthquakes of up to magnitude 9, as well as tsunamis.

Scientists believe the Hikurangi subduction margin has significant earthquake and tsunami potential. In 1947 it generated a magnitude 6.9-7.1 quake and 10m-high tsunami that came ashore in Poverty Bay.

The main aim of 16-day voyage is to learn more about the tectonic deformation mechanisms of the Hikurangi margin so scientists can better understand the earthquake and tsunami hazard it poses.

This margin also experiences slow-slip episodes about every two years. These are small bursts of plate movement to the east lasting a couple of weeks and occurring at depths of 5km to 15km below the seafloor.

Slow-slip events are quiet and are not accompanied by a normal earthquake. If they occurred in a few seconds, rather than a couple of weeks, they would be equivalent to a magnitude 6.3 to 6.8 quake.

Scientists are keen to find out what causes some parts of a plate boundary under Poverty Bay to become locked and release strain very suddenly, while other parts of the plate boundary creep slowly and don’t produce large damaging earthquakes.

They also want to know if the same areas of a subduction fault that experience a slow-slip event can also rupture in a large damaging quake.

The Tangaroa will collect bathymetric data, and seismic reflection and magnetic profiles along grid lines off the Poverty Bay coast. The ship may also collect rocks from the seafloor for analysis.

This high-resolution seismic data acquired on this voyage will help support further international research on slow slip events. The seismic signals will penetrate to about 5km below the seafloor and enable scientists to build a detailed picture of the rock layers hat make up the Hikurangi Margin.

The seismic profiles include three 100km-long lines running across the Hikurangi margin and the Hikurangi trough.