New Zealand: Four and possibly more faults are now believed responsible for the September 4 earthquake in New Zealand. Amongst them, two blind thrusts were angled across the Greendale Fault, according to detailed investigations by GNS Science using satellite radar-imaging, GPS calculations and computer modelling.
The fault to the southwest of Hororata is thought to have ruptured underground third in the sequence, followed quickly by the fourth, around the West Melton-Sandy Knolls-Burnham area.
The scientists said this latter fault helped explained the intense shallow aftershock activity near Rolleston and across to Lincoln.
At this week’s GeoNZ conference in Auckland, GNS seismologist Dr Caroline Holden and colleague, geophysicist Dr John Beavan, outlined their findings.
Holden said the 7.1 magnitude was accounted for by an about 6.3 magnitude quake lasting two to four seconds on the Charing Cross fault, followed by a 6.9 shake lasting seven to 18 seconds on the Greendale Fault and a close to 6.5 jolt of 15 to 18 seconds on the feature near Hororota.
Beavan’s calculations were slightly different, with the initial quake of magnitude 6.5, followed by shakes of 7.0 and 6.2. He believed the last rupture, on the fourth fault, generated a quake of about 6.5.
“We have got more work to do towards the eastern end of the Greendale Fault. Something else is going on there.”
“It was a very complex earthquake. Why was it so complex? Are most earthquakes like this … or is there something special about the tectonics in the Canterbury region?”
The fault near Burnham did not appear in satellite imagery because it was a subtle feature, Beavan said.
“There are probably several bits of fault, little segments that break, but I suspect they are not as important as the other faults. There may be other ones too.”