Monday's quakes reveal new fault
Another fault several kilometres south of the Port Hills fault is now believed responsible for yesterday's major aftershocks.
GNS Science seismologists said the newly confirmed fault had already generated a number of quakes since the deadly February 22 event.
Dr Bill Fry said the dominant energy in Monday's magnitude 5.7 and 6.3 aftershocks had been horizontal compared with vertical in February's 6.3 quake.
This meant they were felt differently.
Unfortunately for Cantabrians, rather than reduce the statistical probability of another big quake, yesterday's violent shakes will, for a time, increase the risk of another large aftershock of similar magnitude.
However, GNS Science hazard modeller Dr Matt Gerstenberger said that elevated level of risk would not last long.
It was also important to realise the risk estimates, and any other aftershock forecasts, were only computer models based on average quake sequences and not derived from any physical evidence, he said.
Only two weeks ago, Christchurch city councillor Sue Wells released aftershock forecast information given to councillors.
The calculations from GNS Science warned of a 23 per cent probability of a quake of magnitude 6.0 to 6.9 hitting the Canterbury aftershock zone within the next 12 months.
GNS Science said the risk of an aftershock of that size occurring under or close to Christchurch was much lower - around 6 per cent.
Yesterday's largest earthquakes were located close to the coast and slightly south of the eastern tip of the Port Hills fault, which generated the magnitude-6.3 quake on February 22.
Given their similar positions, scientists say the 1pm earthquake is believed to be a foreshock of the second quake.
Peak ground accelerations reached 56 per cent of the acceleration of gravity in the first, at Heathcote, and 78 per cent of gravity, at Avonside, in the second.
GNS Science seismologist Dr John Ristau said it would take time for more precise locations and magnitudes to be assigned to yesterday's quakes.
Ristau said after the Queen's Birthday Monday 5.5-magnitude aftershock the trend had been for the large aftershocks to get larger - up from magnitudes 5.0 to 5.1 to 5.3 to 5.5 in recent months. However, there was no evidence that trend would continue.
"Every aftershock sequence is different," he said yesterday.
The quakes would probably kick off a more active spell of aftershocks.
"We can probably expect to get some [magnitude] fours for the next few days, [and] we may have another magnitude 5.0," he said.
Ground on either side of the fault that caused the two aftershocks had slipped sideways to the right and land on the southern side of it had lifted up against the northern side.
Natural Hazards Platform manager Dr Kelvin Berryman , of GNS Science, said the strongly felt shakes were "within the range of forecasted aftershocks as modelled by scientists".
"These aftershocks were within the existing Canterbury aftershock zone and were within probabilities.
"This size of events is likely to produce its own aftershock sequence, therefore rejuvenating aftershock activity at least in the short term. We would expect a number of aftershocks in the magnitude 4.0 to 5.0 range in the coming days and weeks."
Asked if it was the "big" quake forecast as a one-in-four chance in the next year, Berryman said "possibly".
"Whether it is the one or not, this in itself kicks the probabilities again, so we have to go back and recalculate."