Region on shaky ground as 'shocks test science
Last updated 05:00 11/05/2011
Aftershock forecasts issued by GNS Science are being challenged by three large earthquakes in Canterbury in just over three weeks.
The latest, yesterday morning's magnitude-5.3 quake centred about 15 kilometres under Rolleston, was the eighth-largest recorded since the magnitude-7.1 quake on September 4.
The 3.04am shock caused minor damage in Rolleston. It was felt strongly across much of western Christchurch, where its arrival was heralded by loud rumbling reminiscent of last year's quake.
By late yesterday, more than 1800 people had filled in "felt reports" on the GeoNet website from as far afield as Gore and Wellington.
The aftershock was located on the eastern end of the previously hidden Greendale Fault, a hot spot for tremors before the February 22 magnitude-6.3 quake struck and most seismic activity moved east to the new Port Hills Fault.
It followed a 5.2 shake near Lees Valley, northwest of Oxford, on April 30 and another 5.3 aftershock on April 16 centred near Camp Bay on Lyttelton Harbour.
GNS Science's aftershock forecasts say less than one quake of magnitude 5.0 or more is expected, based on averages, from April 19 to May 19. So far, there have been two. Similarly, there should be six of magnitude 4.0 to 4.9. However, eight have occurred, with more than a week until the period ends.
Duty seismologist Caroline Ashenden said the aftershocks were still within the range of the predictions. It was not unexpected that the eastern tip of the Greendale Fault had come back to life.
"We think it might be some structure relating to the old [and extinct Lyttelton] volcano. When the volcano was formed, it had an impact on the rock. There was interplay between the volcano coming up and what it did to the crust."
A visiting geologist from Pennsylvania State University, Professor Kevin Furlong, said there had been a lot of aftershock activity in the "gap" between the Greendale Fault and the Port Hills Fault.
"Once the 6.3 went off, that did its own readjustment of things. But the area where this [yesterday's] one was was very active with aftershocks after the September event.
"This means it still has issues. All these issues are not resolved yet."
Retired Canterbury University geologist and Canterbury faults expert Jocelyn Campbell said nobody could rule out the chance of large events in the zone between the two faults.
"This is a zone of compression and high stress. Whether this means that anything much bigger than last night's event is likely is less sure," she said. "Because events of that magnitude-5.0 size have occurred spasmodically in this zone ever since the beginning, it may simply reflect a cyclic buildup of fluid pressure to a critical value that overcomes frictional resistance.
"It does seem likely that shakes as big as last night's can be expected in this zone for a while yet."
Researchers from Canterbury University and the University of Calgary will soon begin underground seismic surveying of the gap between the two faults.
- The Press