Whose fault is it anyway? The scientists remain divided

Shaken Christchurch residents may believe an earthquake fault lies beneath them, but the question is dividing scientists.

Quakes under Christchurch continue to puzzle some scientists but suggest to others that faults lie beneath gravels directly below the city.

Canterbury University scientists say the pattern of aftershocks since the September 4 quake shows how far east the underground section of the recently revealed Greendale Fault extends.

US visiting geophysicist Professor Kevin Furlong said it appeared to run another 10 to 12 kilometres east of Rolleston – where the surface rupture ends – roughly through the Prebbleton-Halswell area to the Port Hills. There were possibly other faults under the city, but it was not known if they joined up with the Greendale Fault.

Crown research institute GNS Science, which has previously said it does not think a fault runs underneath the city, is still waiting for solid evidence and is suggesting "features" rather than faults.

Natural hazards research manager Dr Kelvin Berryman said there was no proof yet. An onshore and offshore seismic survey that would reveal the truth would cost about $1 million.

The Boxing Day aftershocks and Tuesday's magnitude-4.0 shake were helping build the picture.

"As that new data has come in, we certainly think there are features under Christchurch. There are earthquakes under Christchurch which could be interpreted as an extension of the Greendale Fault, or something with some orientation not dissimilar," he said. "I wouldn't necessarily want to draw a straight line through the city, but with Boxing Day and [Tuesday morning], we are looking to replot things."

Berryman said he was mindful of the nervous state of many in the city and that Canterbury University maps showing the extension of the Greendale Fault were "pretty misleading. I'm still being extremely cautious about that. People in Christchurch are extremely twitchy. Nobody needs to be wound up any more. We don't know if there is going to be a future large earthquake on that structure, but triggering aftershocks on that is absolutely intriguing – rather worrying."

Furlong said the Lincoln-Prebbleton area was where many aftershocks had been centred and where some of the strongest ground motions were recorded. "We know that in general the subsurface fault extends beyond the surface rupture. I'm surprised and don't fully understand why they [GNS] have been so strong about their assumption that the fault ends at depth in the same position as at the surface. That is the most conservative position for it."

Berryman said the fault could extend "a few kilometres to the east of the Greendale surface rupture perhaps, but not much more".

Furlong said it was possible the extension to the Greendale Fault reduced the hazard. "The large number of aftershocks probably indicates a relatively complex fault geometry there, but it is unlikely it could host a big event."

The Press