An expert explains the latest quake.
RNZ/ Sally Murphy
Tumbling rocks, a massive dust cloud, liquefaction in the streets... yesterday's 1.13pm earthquake will have brought back familiar fears for Christchurch residents. And questions. Like, why again, and why here?
GNS seismologist Dr John Ristau told RNZ the 5.7 was yet another aftershock following the 2010 earthquake - it just happened to be a really huge one. "All of this goes back to the September 2010 Darfield earthquake," he said, adding that since 2010 there had been "easily over 10,000 aftershocks".
GeoNet has since done the maths, looking at the probability of aftershocks in the Canterbury region after yesterday's quake, and now says there's a 63 percent chance of 5.0-5.9 quake in the next year - up from 49 percent.
Dr Ristau said yesterday's tremor, which occurred offshore and 15km from Christchurch, was probably on a fault that scientists didn't know yet about. "The problem when it comes to Pegasus Bay, of course, is that everything's under water and it becomes much more difficult to map any faults," he said.
LISTEN: seismologist Dr John Ristau explains the quake to RNZ
"NIWA has done some mapping out there but until we have a chance to look closer at exactly where this earthquake happened we won't know whether it occurred on any of the currently mapped faults although it's most likely that it did not occur on a fault that we knew about."
According to Geonet, the 5.7 quake was the largest seen by Christchurch since a big quake in May 2012, and was on par with what residents felt in the city in December 2011, when a series of strong shocks, including a 5.8, struck.
Yesterday's "peak ground acceleration" measurement was 0.4g - enough for liquefaction to occur - but that's still a lot lower than the 2.2g experienced in the 6.3 February 2011 quake.