There is something people need to know about the seismic faults around Christchurch. We have been in a quiet period for a few years now, since the earthquakes of 2010-11, and no doubt all of you are hoping that it is all over on the Christchurch faults for another several thousand years.
Unfortunately it is not. The disturbing truth is that effectively the Christchurch earthquake sequence is only in a recess of an unknown length of time. Although the earthquakes appeared to be happening all around Christchurch, the major events that defined the sequence occurred on faults around the fringes of Christchurch.
If one imagines Christchurch to be in a crude box, it can be defined by a set of faults that act as the sides. Now imagine the box being a sort of parallelogram being slowly twisted and distorted by an invisible force (plate tectonics). The complex sedimentary geology underneath Christchurch, which is a mixture of alluvial gravels washed down by the Waimakariri River, Port Hill volcanics and marine sediments make up the floor of the box, and Christchurch sits on top of it all.
On 04 September 2010, the western portion of the Greendale Fault ruptured near Darfield in a magnitude 7.1. The rupture extended east to a point a few kilometres from Rolleston. Research infers that the fault however does not stop there, but continues out towards Lake Ellesmere, where it meets the Port Hills Fault. Although the fault runs roughly west-east, the footprint of aftershocks around the fault means it denotes the western side of the parallelogram.
On 22 February 2011, the Port Hills Fault ruptured under the Port Hills in a magnitude 6.3 earthquake. This denoted the south side of the parallelogram. The location of the end of the rupture marked the edge of a gap between the Greendale Fault and the Port Hills Fault, of similar length to the portion that ruptured in 2010. During research done in 2010-11 it was found that an obliquely angled secondary fault merges with the Greendale Fault in this gap. The function of the fault is not known, but one possibility is that it acts as a reliever for stress on the Greendale Fault.
On 13 June 2011, a fault running at right angles and intersecting the Port Hills Fault near Lyttelton, ruptured with a magnitude 6.4. This fault, along with ones further to the east, off the coast which ruptured in December 2011 may help denote the eastern side of the parallelogram.
On 23 December 2011, magnitude 5.8 and 6.0 events 80 minutes apart indicated the presence of faults off the coast from New Brighton. Unlike the 13 June fault rupture there is no suggestion from available data that these faults are connected structurally to each other or to adjacent faults.
That leaves the northern side of the parallelogram, and the portion of the Greendale Fault in the Rolleston gap. One possibility is the Ashley Fault, which has been identified on the surface, and which has been the subject of investigative work by local councils. Another fault is the Cust Fault, which is thought to be capable of a magnitude 6.4 earthquake. Other onshore faults are known to exist around Kaiapoi, but have not been identified.
Enjoy the peace while it lasts because we do not know when the seismic recess will end.