16 March 2011

NIWA’s research vessel Kaharoa set sail from Wellington today, destined for Lyttelton and equipped with seismic survey gear to survey an area of southern Pegasus Bay.

The seismic survey work being undertaken by NIWA is part of Project Offshore, a part of the effort by the Natural Hazards Research Platform in supporting Response and Recovery to the Christchurch earthquake. The survey results will assist in the understanding of future earthquake in the Canterbury region, in preparation for rebuilding the city.

The scientists will identify whether there are any faults in the coastal marine area, that could lead to undersea earthquakes off the Canterbury Coast. The survey will help with understanding the broader fault zone along which the Christchurch faults have ruptured.

NIWA’s principal scientist and voyage leader Dr Philip Barnes says that the scientists “are trying to identify if there are any faults that represent, or are part of, this fault zone, that potentially extends off the coast in the Banks Peninsula area.

On Thursday, the scientists will begin a seismic survey of southern Pegasus Bay, in the Banks Peninsula area. They will survey out to 40 km east of the New Brighton Coast, up to 30 km north of Lyttelton heads. The six scientists, including an Otago University scientist, and six crew will be at sea for up to 7 days.

The scientists are determining what is in the area, and the likelihood of rupture occurring on an offshore fault.

“We don’t know what we will find. We could find no faulting. There is no evidence for a fault zone off shore, based on existing information,” says Dr Barnes. The area has not yet been comprehensively surveyed, and there is very little marine geological information around Christchurch, Banks Peninsula, and Sumner. Dr Barnes conducted the last survey work in Pegasus Bay nearly twenty years ago.

On board Kaharoa, the scientists will be collecting seismic reflection data using multi-channel seismic gear and a seismic boomer. Both seismic acoustic systems project sounds down into the water and then look at the sounds reflected back from the seafloor and from layers (immediately) underneath.

The seismic boomer is designed for looking at shallower depths, up to 100 metres, with high resolution. “If there was a fault that had ruptured up to the sea floor, we can see it in the seafloor images,” says Dr Barnes.

“The multi-channel seismic equipment enables us to look deeper into the sea floor - 1 to 1.5 kilometres down, to see any hidden faults, buried beneath the sea floor, that we wouldn’t see with just the surface imagery,” says Dr Barnes.

The combination of multi-channel and seismic boomer enables the scientists to look at the upper 1.5 km zone, at faults that haven’t been activated or reactivated, for some time. “If the fault has not ruptured in the past tens of thousands of years, we won’t see anything,” says Dr Barnes.

The faults that have been rupturing in the recent Christchurch earthquakes are very old faults that have been reactivated.

Dr Barnes says that if the scientists “find a new fault, we will try and focus in on that to map it accurately. The new data will be combined with data held by GNS Science and partners - to look at the likelihood of future events.”

By merging this information with similar records from on land, the scientists can learn how the different faults relate to each other. A history of ‘when, where and how often’ can be developed for each fault. This can be used to model earthquake risk.

ENDS

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