New research has uncovered the role underground rock structures played in stopping the September 2010 Darfield earthquake from linking up with the Port Hills faultline and causing greater damage.
The discovery has been made by scientists from New Zealand and the University of Southern California.
Lead US researcher Yong Gang Li says the 7.1-magnitude earthquake was not strong enough to leap over the rock barrier to the Port Hills fault.
Mr Li says this is important because longer fault lines produce stronger quakes.
He says if the two faults had joined up, they could have produced a 7.3-magnitude quake.