The location and scale of potential faults under the Canterbury Plains must be understood before Christchurch pours billions of dollars into rebuilding, a geologist told the New Zealand Herald.

Both the September earthquake and last week's deadly tremor occurred on fault lines that did not exist on GNS Science's database.

University of Canterbury geologist Mark Quigley said further ’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,” he told the paper.

Quigley said cities on similar fault line networks, such as Los Angeles, had done three-dimensional seismic surveys, and these informed their engineering decisions.

While aware of the need to tread sensitively after the quake, he believed it was a matter of urgency.

Quigley added that funding would be relatively inexpensive at around $1m and could map fault lines to within 100 metres.