Two shallow quakes that shook Christchurch this afternoon 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.
St John spokesman Ian Henderson said there had been no reports of injuries or any calls as a result of the quakes.
However, St John was encouraging everyone to check on their neighbours and family as this had been the first widely felt and reported strong quake Christchurch has had in over a month.
GNS duty officer Martin Rayners said today's quake was 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.
The area of active faulting was identified in an undersea survey last year.
Rayners said today's shakes were part of "the ongoing readjustment process" in this area and "nothing to worry about".
"It's still in the aftershock period of that December event," he said.
"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.
Twitter user Kineta Knight tweeted: "I actually had a look out the window because I thought a truck had slammed into the building."
Another user, Carly McCall, wrote "that was pretty Ka-BOOM here in Sydenham, on the second floor", while Alan Smith tweeted: "Four storeys up when you feel the first decent shake in months = unpleasant!"
Donna Robertson tweeted: "I was writing about Christ Church Cathedral when that little bruiser made my desk (and heart) lurch."
Dale Small wrote: "Felt that wee one over here in Greymouth."
A Canterbury Earthquake Recovery Authority spokeswoman said there was "not a whisper" of reported damage caused by the quakes.
More than 500 people reported they felt the 1.35pm quake to GeoNet.
Of those, more than 480 said it felt "light", while about 20 thought it felt "moderate" and about 10 said it felt "weak".
The last widely felt quake across the city was an 8km-deep, 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.
- The Press