Cook Strait quakes too small for landslide-tsunami
A seabed survey in Cook Strait by NIWA’s flagship research vessel Tangaroa showed that the origin of the magnitude 6.5 July earthquake was a previously unknown ‘blind’ fault.
Within three days of the quake, the vessel headed to Cook Strait to survey the seabed using a multibeam echosounder. It surveyed an area crossing the epicentre to determine whether there were changes to the seafloor due to near- surface faulting.
The survey, which remapped the deep canyon system, supported research by NIWA Ocean Geologist Dr Joshu Mountjoy that showed the level of ground shaking generated by this earthquake was not sufficient to trigger submarine landslides – which pose a tsunami hazard in the area.
“This earthquake occurred near one of the least stable regions of Cook Strait Canyon,” says Mountjoy. “Despite the significance of the event in Marlborough and Wellington, it was well below the level of ground shaking required to trigger sediment failure.”
Mountjoy’s research on submarine canyons, soon to be published in a special issue of the journal Deep Sea Research Part II, shows that about every 100 years a landslide in the region will displace 50 million cubic metres of seabed – more than four times the size of the landslide on Mt Cook in 1991.
“While the landslides focused on in this study were too small and far below the sea surface to cause tsunamis, they play an important role in the development of the canyon. There have been landslides ten times bigger within the canyon walls, and we are still working to determine the hazard risk related to them,” he says.
The Cook Strait Canyon starts just 10 kilometres off Wellington’s south coast and plunges from a depth of 150 metres to 2700 metres south of Cape Palliser. The canyon is the most prominent undersea feature of Cook Strait and is scarred by numerous large submarine landslides and active faults.
Mountjoy’s research paper is based on data collected on board Tangaroa in February 2011, and measurements of ocean currents in 2006. He says early indications from the July 2013 survey back up the assessment of how sediment falls and moves within the Strait.
This work identifies that sediment is transported into the canyons by the strong tidal currents that scour the seafloor of Cook Strait. Sediment is accumulating in the central axis of Cook Strait Canyon at 400 metres water depth – the least stable region of the canyons. Landslide deposits in a sediment core sample taken further down the canyon support the modelling-based estimate of how often landslides occur.
“Soft sediment landslides flow down the canyon, eroding the canyon floor, much like a river in flood carrying sand and gravel,” says Mountjoy. “This type of erosion is the main mechanism by which canyons form, and is also very important to biological communities,” he says.
This work forms part of a larger, ongoing Natural Hazards Research Platform project by NIWA and GNS Science focused on determining the hazard posed by landslide tsunamis in Cook Strait.
Tangaroa had been stationed off the Marlborough coast mapping an area of Pegasus Basin as part of a Ministry of Business, Innovation and Employment-funded (MBIE) Ocean Survey 20/20 project. NIWA acknowledges the cooperation of MBIE in enabling this timely diversion.