4.1-magnitude aftershock in Christchurch

11:33 am on 25 May 2011

A significant aftershock in Christchurch cut power supplies to several suburbs for a short time on Wednesday morning.

The 4.1-magnitude tremor struck at 6am on Wednesday, at a depth of 7 kilometres.

GNS Science reports it was 10 km south-east of Christchurch and within 5 kilometres of Lyttelton.

Power was out in the Heathcote Valley, Woolston, and parts of the eastern hill suburbs for about 20 minutes after the quake tripped a substation.

GNS Science seismologist Bill Fry says the quake had quite a large impact because it was so shallow and because the valley has already been so badly affected by the quakes.

"The area around Heathcote Valley ... has experienced a lot of shaking, a lot of side amplification, so even a distant earthquake has been felt more strongly than other places."

Dr Fry says Wednesday's tremor is a normal part of the process of aftershocks.