In brief: Kaikoura tsunami hazard

NIWA Ocean Geologist Dr Joshu Mountjoy is checking the validity of a theory that a massive undersea landslide could hit Kaikoura with a 13-metre-high tsunami.

The effect of submarine landslides is very real. Devastating tsunamis generated by submarine landsides occurred on the Grand Banks off Newfoundland in 1929 and Papua New Guinea in 1998, killing 28 and 2200 people respectively.

A 1999 scientific paper (Lewis and Barnes) posed that a large amount of sediment had recently accumulated in the head of Kaikoura Canyon. Work published by Walters et al. (2006) showed that collapse of this sediment would cause a large tsunami.

The canyon feeds sediment directly into the 1500-kilometre-long Hikurangi Channel, which sweeps along the east coast of New Zealand 400 kilometres east of East Cape. The canyon is up to 4000 metres deep and comes to within 500 metres of the coast south of Kaikoura, closer than any other New Zealand submarine canyon.

So Environment Canterbury (ECan) asked NIWA to resurvey the area in March this year, using state-of- the-art techniques to determine if the submarine landslide scenario is realistic.

"We spent three days surveying the head of the Kaikoura Canyon, mapping the seabed, imaging the sediment and rock beneath the seabed and sampling the sediment," Dr Mountjoy said.

The survey used three different techniques.

A hull-mounted echosounder imaged approximately eight square kilometres of the seabed.

"The bathymetry and backscatter data enable us to assess previous landslide history and to map the distribution of surface sediments."

A multichannel boomer seismic refection system was used to 'see' 50 metres into the sediment and rock below the seafloor.

"We are able to unravel the history of sediment deposition and erosion in the area during the last 20,000 years."

The survey also used two coring techniques to collect seven seafloor sediment cores up to one metre long.

"We are analysing these to determine the rate at which sediment is accumulating on the seafloor on modern (last 100 year) timeframes."

The project team is now assessing the data collected over the three days to learn more about the way the sediment moves and accumulates in the canyon, and whether a landslide is realistic.

It will report to ECan later this year.