New fault found below Canterbury

The pace of building demolition approvals in the Christchurch CBD has slowed, and the cordon around the centre of the city has shrunk. Photo / Sarah Ivey

A previously undiscovered fault capable of generating a magnitude-7 earthquake has been found by scientists investigating bedrock off Christchurch.

Analysis of the Canterbury earthquakes is being carried out by teams of scientists from agencies including GNS Science, NIWA and the University of Canterbury.

They have found a previously unknown "complex arrangements of faults" in the bedrock under Canterbury and offshore under Pegasus Bay.

One of those was an offshore fault about 25km long - similar in length to the Greendale fault which caused the original Canterbury earthquake on September 4.

The large fault was discovered when NIWA's research vessel Kaharoa conducted a seismic survey of 800 square kilometres in southern Pegasus Bay.

NIWA principal scientist Philip Barnes said it was part of a widespread network of faults in basement rocks in the bay.

Most were millions of years old and "very slow moving" - though a small number of faults showed evidence of reactivation in the more recent geological past, he said.

"There are younger fault structures that are active and they are reactivating the older faults."

However, he said there was no strong evidence of aftershocks in the region migrating toward Pegasus Bay at present.

"Understanding the earthquake hazard for the coming years and decades requires an assessment of all the faults in the region, and learning about their rates of activity and earthquake potential."

University of Canterbury professor Jarg Pettinga, who led the onshore seismic surveys, said his team's data had revealed several previously unknown fault structures, mostly trending northeast-southwest.

They were likely to be the bedrock structures on which the magnitude-5.3 Boxing Day and magnitude-6.3 February 22 earthquakes occurred, he said.

"They appear to be very old faults in the basement rock that may have been weakly reactivated. Cantabrians live in a seismically active area, and finding previously unknown faults that seem to have very long intervals between ruptures won't have significantly increased the level of earthquake hazard in the region."

It was previously announced there is a 23 per cent chance of a magnitude-6 to 7 earthquake striking Canterbury in the next year.

There is also a six per cent of that aftershock being centred under the city of Christchurch.

Kelvin Berryman, manager of the natural hazards research platform, said even before the investigation started, it was widely known that there were several dozen active faults within an 80km radius of Christchurch and many were capable of producing a damaging earthquake.

Most of the newly discovered faults had their major period of movement prior to about 50 million years ago, he said.

"The focus of the investigation is to identify the length, orientation and spatial arrangement of faults in the aftershock zone in Canterbury.

"For an earthquake of magnitude-6 or greater to be generated, a fault of 10km or more in length is needed."

A total of 182 people were killed in the magnitude-6.3 quake that struck Canterbury on February 22.