4.6 quake centred in 'The Gap'
Cantabrians woken by yet another aftershock are being reassured that the shake was part of the expected sequence.
The late night jolt came at 11.36pm on Monday, and was initially recorded as magnitude 4.4.
GNS Science later upgraded this to 4.6 and located the aftershock 10km south-west of Christchurch, between Halswell and Prebbleton, at a depth of 8km.
The region southwest of Christchurch, around Prebbleton and Lincoln, has been dubbed The Gap.
It is a physical void between the east of the Greendale Fault, which ruptured in September 2010, and the western end of the Port Hills Fault, which caused widespread damage in February 2011.
GNS Science seismologist John Ristau said an aftershock in that region was not unusual, and fit with the pattern of aftershocks.
''It's not so much that the earthquakes get smaller and smaller with time,'' he said. Rather, ''they get less frequent''.
Though in the early days, Cantabrians were shaken by many magnitude four and five earthquakes a day, now they occur ''months apart,'' Ristau said.
Whether there was still energy to be released from The Gap was ''a matter for debate,'' Ristau said.
University of Canterbury geologist Professor Jarg Pettinga said that while the shake was the ''first significant event for quite a while,'' there was nothing surprising about an aftershock occurring in the area.
''It's an area which has been quite rich in relatively small and moderate sized aftershocks,'' he said. ''There's nothing unusual about it from that point of view.''
Nearly 4000 ''felt reports'' were logged with GNS in the 12 hours following the aftershock, including from Wellington, Dunedin and the West Coast.
"I think everyone will just be hoping there won't be more to come," said Tracy Haley, of Hillmorton.
Arvind Chandna, currently visiting Christchurch on holiday, said he felt "a big shake" in Sumner.
"It made one of us get out and collect our belongings and car keys ready to move out," he said.
"The coat hangers in the wardrobe shook and swayed and the bed moved with the earth."
Rebecca Cousins, a Cantabrian living in Germany but currently visiting family in Christchurch, said she "woke myself up in the doorway".
"Good to know that subconsciously I still know to do something when one hits."
Sarah Willis said she definitely felt it in Hoon Hay and knew it was close by. "It was very loud and took me right back to that September quake."
She said she was surprised to find family in Woodend and Rangiora also felt it.
"Usually they don't feel them but this time there seems to be a lot more power behind it that your usual 4."
Alec B, visiting New Zealand from the UK, said he had coincidently been to the Quake City exhibition in Re:Start Mall earlier in the day.
"[It was] a bit scary to be woken up . . . when the whole room was shaking," he said.
"The wife wanted the earth to move for her but not quite like that!"
Jenny Morgan, who lives in a new build in Halswell, said she was reading in bed when the quake hit.
"We have said several times how nice it is to live in a house that has not been shaken and rocked time after time," she said.
"To me it felt more of a rolling shake and went on for some time. I haven't checked for breakages yet, although the house creaked quite a bit, it is built on the new rib-raft method of foundation which is now used in all new houses since the earthquakes."
Jan Cole in Diamond Harbour said the quake roused her from her sleep.
"I got out of bed, found boots and torch and checked my house," she said.
"No breakages that I can see.
"[It] felt like a 5.0 . . . cats don't usually scatter for a 4.0 but they did for this one."
In Halswell, Kay Skerten said she knew she was "very close" to the centre of the quake and felt it strongly. "I headed out of bed to the door."
She said the shock was noisy and "had The Gap feel about it" with the sound of rocks "grinding underneath".
Helen Albrow, visiting from Coventry in the UK, said she was woken "by the realisation that the earth below me was moving. It was scary".
"It simply doesn't happen back home. I've only felt two other quakes in my 29 years. One in California (around a 3) and one back home (around two-three years ago)."
Did you feel the quake? Did it get you out of bed? Email firstname.lastname@example.org