Canterbury quake may have been on 'new' fault line

Rubble lies in the street after this morning's 7.1 magnitude earthquake. Photo / Getty Images

Geologists say today's 7.1 magnitude quake 40km west of Christchurch city had an epicentre 10 km south-east of Darfield, but so far they have not been able to link it to a known fault line.

"At this stage it appears the earthquake has not occurred on a known fault," said GNS Science duty seismologist John Ristau.

In the past few years, researchers have found that Canterbury is riddled with more earthquake-generating fault zones than previously thought - some as close as 20km to central Christchurch.

Canterbury University scientists have said there are about 100 fault lines and fault segments around the region, rather than the half-dozen active faults that were known about 20 years ago.

The closest known faults to Christchurch capable of generating powerful quakes are in the Rangiora-Cust area, near Hororata, and near Darfield.

Regulators investigating Central Plains Water's proposed Wairiri Dam in the area were told that the Hororata Fault "probably generates earthquakes of magnitude 7 or more every few thousand years".

That fault line passes about 6km south of the proposed dam site and under its head race.

The Hororata fault between Darfield and Sheffield can be seen on the land surface because of the warping of alluvial plains, and other fault lines have been recognised between Darfield and Porters Pass.

Canterbury University geological sciences department head Jarg Pettinga has said scientific advances in recent years have enabled the detection of many faults in Pegasus Bay, under the Canterbury Plains and through the foothills of north and central Canterbury.

The department's active tectonics and earthquake hazards research programme had gone from recognising half a dozen active faults to realising to knowing there are now 100 earthquake-source structures.

As an example, 20 years ago geologists knew about the active Porters Pass fault but had no knowledge of the timing of quakes it generated or their size. Now they know it last moved between 500 and 600 years ago and has ruptured six times in the past 9000 years, causing earthquakes of between 7.1 and 7.7 on the Richter scale.

Earthquakes generated by several active faults in the Rangiora-Cust area are estimated to have been about 6.8 or 6.9.

The university's initial research several years ago found that though Christchurch city was apparently outside the plate boundary zone, the deformation from movement of tectonic plates was happening right on its doorstep.

But the researchers said faults closest to the largest population centres appeared to have thousands of years between movements, rather than the hundreds of years of the much larger Alpine Fault and the Hope Fault.

A 2006 earthquake hazard and risk assessment study carried out by Opus International Consultants Ltd for the Canterbury regional council listed historical quakes around Christchurch:

* 1869 Christchurch magnitude 5 (epicentre Addington)
* 1870 magnitude 5.5 (epicentre Lake Ellesmere)
* 1881 Castle Hill 6.0 magnitude (epicentre Cass)
* 1888 North Canterbury 7 - 7.3 magnitude (epicentre Hope Fault, west of Hanmer Springs)
* 1901 Cheviot 6.9 magnitude (epicentre Parnassus)
* 1922 Motunau 6.4 magnitude (epicentre Motunau/Scargill)
* 1929 Arthurs Pass 7.01 magnitude (epicentre Kakapo Fault at Arthurs Pass)
* 1929 Buller quake 7.8 magnitude
* 1994 Arthurs Pass 6.7 magnitude (epicentre Arthurs Pass)

- NZPA