The location and scale of potential faults under the Canterbury Plains must be understood before Christchurch pours billions of dollars into rebuilding, according to University of Canterbury geologist Mark Quigley.
Both the September and February earthquakes occurred on faultlines that did not exist on GNS Science’s database.
Dr Quigley says further unknown or “blind” faults could lie under Christchurch’s soil, and priority should be placed on creating an ultrasound-like image of the city’s subsurface.
“We’ve so far been struck by two faults we didn’t know about. So here’s the question: is there a fault that’s really short but capable of a magnitude-four earthquake in the immediate Christchurch area? This can be answered. And we need this data before we even talk about rebuilding.”
Dr Quigley says cities on similar faultline networks, such as Los Angeles, had carried out three-dimensional seismic surveys which influenced engineering decisions.
While aware of the need to tread sensitively after the quake, he believes it is a matter of urgency, and says would be relatively inexpensive — around $1 million — to carry out geophysical tests under the city.
Studying aftershocks gave some clues to the potential for faults under Canterbury, but this could be reinforced by seismic surveys as the combined studies could map a faultline to within 100 metres.
The most common method of surveying is to use a truck-mounted device, commonly used for oil and gas exploration, which pounds the earth.
Instruments called geophones are placed alongside a truck-mounted device that pounds the earth. They measure the strength and angle of the vibration when it returns and this data is used to map the earth’s crust.
Because of the costs and complications of the process, scientists have mostly focused on mapping areas where there is a surface rupture or known activity.
University of Otago geologist Andrew Gorman says geophysical surveys were helpful, but complicated. He says some surveys taken in the Canterbury Plains in September did not provide a convincing image.
Dr Gorman recently worked on mapping parts of the alpine fault, and says measuring the faultlines in the foothills of the alps was relatively straightforward, because of the clear distinction in rock on either side of the fault.
However, this was not the case under Christchurch.