By Susan Sandys  September 9 2010

Geologists have both good and bad news for Mid Cantabrians rattled by Saturday's earthquake and continuing aftershocks.

The faultline which triggered the quake does not extend into the district, but the initial earthquake could result in aftershocks centred this side of the Rakaia River.

GNS geologist Simon Cox said he and fellow GNS staff had traced the faultline near Darfield, where the 7.1 quake was centred, from a helicopter.

"It stops pretty much at the Selwyn River, we haven't managed to track it any further south," he said.

The Selwyn River is about 35km from the Rakaia Gorge bridges.

Dr Cox said yesterday's 5.1 magnitude quake at 7.50am centred near Diamond Harbour may not be activity on a separate faultline, but instead a fracture or crack through rock formations that had been twisted as a flow-on from Saturday's big quake.

There was no guarantee such aftershocks in coming weeks would not be centred this side of the river.

"I think it's fair to say there will be a whole range of different things go on as the (earth's) crust recovers from this, it would not surprise me at all if you got small aftershocks and things happening all around the area, but I would not want to make predictions because they are just so unknown," Dr Cox said.

He said the quake had caused some rockslides on the lower eastern face of Mt Hutt, and a "spectacular" landslide in an aftershock he had photographed from the air on the Rakaia River, about five kilometres upstream of the gorge bridges.

Rockslides into the river had not changed its course, but had made it muddy.

Geologists were currently surveying mountains, including Mt Hutt, to see if they had shifted in the original quake.