Christchurch aftershocks links probed

The possibility of a link between two shallow aftershocks which shook Canterbury yesterday is being pondered by seismologists. Photo / Thinkstock

The possibility of a link between two shallow aftershocks which shook Canterbury yesterday is being looked into by seismologists.

The quakes, one magnitude 4 and the other 4.7, occurred 15 minutes apart, but on different fault lines.

The first, a magnitude 4.0 earthquake, struck near Camp Bay in Lyttelton Harbour on the Port Hills fault line at 1.15pm, at a depth of 5km.

At 1.30pm, a magnitude 4.7 earthquake followed, this time 20 kilometres south west of Christchurch, near Rolleston, on the Greendale fault line, at a depth of 11km.

It was described in a tweet by Hayden East as "a long wobble with a hard jolt in the middle".

Duty seismologist Brian Ferris said there had been no confirmation yet the first tremor triggered the second quake.

"The second event, if it was ready to go, it could well have been triggered by the first one. It's just a possibility."

Pennsylvania State University seismologist Professor Kevin Furlong told Fairfax Media the quakes were hard to explain.

He said either the first quake changed the "stress field" such that the second one was more likely, or "a second type of triggering is caused by the actual passage of seismic waves, usually surface waves, from the first event locally changing the stresses, making conditions right for the second".

Mr Furlong said the second explanation was unlikely as the second quake occurred after seismic waves from the first jolt were over. However he said the first explanation was equally unlikely, as the magnitude 4 quake was too small to have changed the stresses.

Mr Ferris said the quakes comply with GNS Science's expectations and have not changed their forecasts for future earthquakes in the Canterbury region.

According to GNS' last forecasts, updated last month, there is a 26 per cent chance of up to two quakes magnitude 5 to 5.4 occurring between August 15 and September 14.

Within that same period there is a 6 per cent chance of a magnitude 5.5 to 5.9, a 3 per cent chance of a quake 6 to 6.4, and less than one per cent chance of a magnitude 6.5 and higher.

In the next year, there is an 82 per cent chance of a up to five quakes measuring between 5 and 5.4, a 39 per cent chance of up to two quakes 5.5 to 5.9, a 10 per cent chance of a quake between 6 and 6.4, a 5 per cent chance of a magnitude 6.5 to 6.9 and a 2 per cent chance of a jolt measuring 7 to 7.9.