Two shallow quakes that shook Christchurch yesterday were aftershocks of the December 23 quakes.

The first, a magnitude 4.3 quake, struck at 1.35pm 15 kilometres east of Christchurch and was 12km deep.

The second, a magnitude 3.6 shake, hit at 2.01pm, 15km east of Christchurch, and was 11km deep.

GeoNet classified the first tremor's intensity as "strong" and the second as "moderate".

The shaking from the first quake lasted at least 10 seconds in the central city, with social media users saying it was felt widely across the city.

There were no reports of injuries or damage as a result of the quakes.

GNS duty officer Martin Rayners said yesterday's quakes were an "expected" result of the December 23, 2011, quakes.

A patchwork of faults below Pegasus Bay about 10km off New Brighton triggered the pre- Christmas quakes last year, he said.

The area of active faulting was identified in an undersea survey last year.

Rayners said yesterday's shakes were part of "the ongoing readjustment process" in this area and "nothing to worry about".

"The whole sequence since September 2010 has been prolonged. We knew these quakes would continue to happen for some time."

He said the quake at 2pm was an aftershock of the 1.35pm quake.

"That 4.3 has its own little aftershock sequence.

"Some people, particularly near the coast of New Brighton, may feel some of these small aftershocks in the magnitude 3 range over the next day as a result of that," he said.

The last widely felt quake across the city was an 8km-deep, magnitude 3.7 tremor on August 9 at 8.03pm.

There was a 4.4 shake on July 10, followed by a 4.9 quake the same day.

- © Fairfax NZ News