Scientists analyse aftershocks

Sunday night's large aftershock may prove important in determining whether more damaging quakes may hit Christchurch.

The magnitude-5.1 quake at 9.47pm – located about 10 kilometres under the city's eastern suburbs – was followed by several small aftershocks in the area. Some of these have been analysed and others have yet to be evaluated by GNS Science.

Some scientists believe the Port Hills fault may have been responsible for the magnitude-4.9 Boxing Day quake, although they say it is too early to draw conclusions. They expect to have a clearer picture after more aftershocks.

GNS Science researchers had said the December 26 quake was on a fault under central Christchurch, slightly further north but roughly parallel to the Port Hills fault that caused last month's 6.3 quake.

The importance of the Boxing Day fault has been highlighted by United States geophysicist Kevin Furlong.

Two weeks ago he said it might still be a threat to Christchurch and generate another magnitude-5 quake.

But was that Sunday's night shake?

Furlong said yesterday it was "tempting" to say that quake and its aftershocks had been on the eastern end of the Boxing Day fault.

"This event [like the Boxing Day event] is likely to have some aftershocks associated with it, so in a couple of days we will have a better sense of whether it defines a linear continuation of the fault that was `lit up' on Boxing Day.

"There is nothing in its location and/or magnitude that makes it any more of a concern than other aftershocks in the region though."

Maps from the US Geological Survey showed most of the February 22 aftershocks occurred on the east-west oriented fault, but the Boxing Day aftershocks and the fault highlighted by Sunday night's quake appeared more east-north-east, Furlong said.

"This is worth watching," he said.

GNS Science seismologist Bill Fry said he was also looking at whether there was a relationship between Boxing Day and Sunday night "but it's a bit premature to draw conclusions".

It was possible the Boxing Day fault and the one that caused Sunday night's quake were in line but did not join up, he said.

Canterbury University geologist Mark Quigley said he doubted it was the same fault.

"To me the locations are different, different enough that they can't be put on the same fault plain."

GNS duty seismologist Martin Reyners said the 5.1 aftershock was probably caused by rocks slipping about 10 centimetres along a fault up to 3km long.

The Press