Christchurch earthquake aftershocks puzzle scientists
Last updated 05:00 12/03/2011
Frazzled Christchurch residents are being rocked by sizeable aftershocks far more often than forecast.
Since last Saturday morning there have been nine quakes of magnitude 4 or higher.
Based on average aftershock decay rates after the February 22 magnitude-6.3 quake, there should only have been two, and at the outside three, during the week.
GNS Science seismologist Matthew Gerstenberger said the aftershock sequence did not indicate what would happen next. Nor had the risk of a magnitude 5 shake changed as a result of all the magnitude 4 aftershocks.
GNS has already said the largest aftershock of the week, the 4.8 shake last Saturday at 7.34pm, could end up being re-analysed as a magnitude 5.
"There's definitely been more than we would expect for this last week," he said.
"Without a doubt the first day [after February 22] was very active, but it settled down really quickly after that, much more quickly than we would have expected given the activity in that first day. But now this week it has been higher than the average of aftershocks we would expect."
The latest four-week forecast of aftershocks from GNS says that from March 8 to April 4 there should be between four and 15 quakes of magnitude 4 to 4.9 and between zero and three of magnitude 5 and above.
Canterbury University physics postgraduate student John Holdaway's update of how much energy has been released by the quakes since February 22 shows how active the ground is under Christchurch.
He said the 6.3 quake released about 180 trillion Joules (J) of energy, about 16 times less than the 2.8 quadrillion Joules of the 7.1 event last September. Aftershocks since February 22 had released a further 87 trillion Joules, or about half the amount as the 6.3 quake, much of that in the large 5.7 and 5.5 magnitude shakes in the two hours after the main quake.
However, the total energy from the 6.3 quake and all aftershocks since only accounted for 8.3 per cent of all the tectonic energy released since September 4.
Despite that, Holdaway said more than two-thirds of the aftershock energy released since September 4 had been given out since February 22, with almost half of the total aftershock energy released since September 4 contained in the 6.3 quake.
University and Crown Research Institute scientists are now planning underground surveys to discover if other faults lie below Christchurch. Scientists and engineers studying the effects of the February 22 quake are asking for more people to fill in a GeoNet questionnaire on how the quake affected them and the building they were in when it struck.
GNS Science civil and structural engineer Andrew King said it was understandable that completing an online questionnaire was a low priority for many people straight after such a damaging quake.
The questionnaire can be found at www.geonet.org.nz/christchurch.
- The Press