More shakes for a while but forecast unchanged

Photo / NZPA

Christchurch's aftershock sequence has been re-energised but not reset by Monday's two large earthquakes.

The magnitude 5.7 and 6.3 aftershocks were connected to the two threads of aftershocks rumbling under Canterbury's soil since the massive September and February tremors.

GNS Science hazard modeller Matt Gerstenberger said the most recent earthquake would "reinvigorate" the sequences, creating a temporary cluster of magnitude 4 and 5 quakes in the coming weeks.

But he stressed that the quakes "do not take us back to year zero".

"The main feature here is the 7.1 sequence [from the Darfield quake in September].

"Monday's quakes do not have a significant effect."

In two months, the chance of a magnitude 6 quake in Canterbury will have dropped to 20 per cent.

In nine months, residents in the region will still experience at least one magnitude 4 quake a month.

After the February quake, seismologists predicted related tremors would continue under Canterbury for around two years.

That forecast remained intact, because only the short-term activity has been affected.

The scale of the aftershocks has been described as unusual by seismologists, who are still investigating why such a rich vein of shocks has developed compared with their forecasts.

Before Monday's quake, GNS Science had said there was a 25 per cent chance of a magnitude 6 or 7 quake in Canterbury, and a 6 per cent chance in Christchurch.

The multiple strands of aftershocks running concurrently in similar territory has complicated forecasts.

The most recent tremors, which occurred at 6km and 11km depths near the coastline, have not placed any extra burden on Canterbury's faultlines, including newly discovered ones.

GNS Science, Niwa and the University of Canterbury found three new faults under Pegasus Bay, Hagley Park and the Port Hills in research conducted after the February quake in Lyttelton.

While large tremors can add stress to surrounding faultlines, they can also remove energy from the crust.