MEDIA RELEASE

November 29, 2013

Selwyn, Waimakariri earthquake fault reports released

Environment Canterbury today released the fourth and fifth in a series of earthquake fault reports on different districts of the Canterbury region.

Environment Canterbury Commissioner Donald Couch says the Selwyn and Waimakariri districts are priority locations for fault survey. “These areas have experienced unprecedented growth since the 2010 and 2011 earthquakes,” Mr Couch said. “It is therefore important that we understand the risk profile.”

Most of the faults and folds identified on the ground surface in the two districts are in rural or very sparsely populated areas, and most of the faults have a relatively long average time between fault movements; in the order of several thousand years.

The report, by GNS Science, provides information on the locations and character of active geological faults and folds throughout the Selwyn and Waimakariri districts.

The faults are mapped at a regional scale. The mapping identifies areas where narrow zones of very intense earthquake deformation could happen, with ground fracturing along the fault and permanent ground offsets on either side of the fault (uplift and subsidence and/or horizontal movement along the faultline).

Sometimes the ground surface will buckle rather than break – this is known as “folding”. These types of deformation - fault rupture and folding - are separate hazards from ground shaking and liquefaction which (as Cantabrians well know) may also happen with earthquakes, but they affect a much wider area.

“The GNS report is intended to highlight areas where there is a risk of faults cutting the ground surface, and where more detailed investigations should be done if development is proposed in the area,” Mr Couch said. “This mapping is not precise enough for site-specific assessments however.”

Mr Couch says Environment Canterbury and Selwyn and Waimakariri district councils will use the report for land information requests (LIRs), land information memoranda (LIMs), public education, and planning and policy. “The information will also be useful for general emergency management planning, and for lifeline utility and infrastructure planning,” Mr Couch said.

Background

The series of earthquake fault reports by GNS Science provides up-to-date, region-wide information on where active faults cut the ground surface, and how active those faults are.

When a fault ruptures or buckles the land surface, any buildings or infrastructure directly above or nearby can be damaged by being tilted or wrenched in different directions.

The information in the latest reports can be summarised as below. A “recurrence interval” is the length of time that passes, on average, between events when the fault breaks through and moves the ground.

Although there are several active faults in the Selwyn and Waimakariri Districts, the probability of a fault rupturing within these districts is low. In any year, there is a 1% or less chance of a fault rupturing somewhere within the district. And for residential areas it is 10 times less likely again.

In Selwyn:

· Most “definite” or “likely” faults are in sparsely populated areas.

· Two towns, Sheffield and Hororata, have “likely” faults nearby, which are yet to be confirmed.

· There are no known faults near Lincoln or Rolleston.

· The Greendale fault, north of Rolleston, was mapped in detail in 2010/2011. This fault is thought to have a recurrence interval of several thousand years.

· Most Selwyn faults are estimated to have a recurrence interval of several thousand years. Only three have shorter intervals - the Porters Pass, Harper and Acheron faults in the west of the district.

In Waimakariri:

· Most “definite” or “likely” faults are in sparsely populated areas.

· Two towns, Rangiora and Oxford, have “likely” faults nearby, which are yet to be confirmed.

· There are no known faults in Kaiapoi.

· Most Waimakariri faults are estimated to have recurrence intervals of several thousand years. Only four have shorter intervals - at Lees Valley, Starvation Hill, Ashley-Loburn and Knowle’s Top. The most active is Knowle’s Top, which still has an active recurrence interval of a few thousand years.

· The Ashley-Loburn Fault, north of Rangiora, which is estimated to have a recurrence interval of several thousand years, is the subject of a more detailed study that will be released later this year.

Earlier reports covering the Hurunui, Ashburton and Mackenzie districts, the last-mentioned including detailed information on the Ostler Fault at Twizel, were released in 2009, 2010 and 2012 respectively. A separate report on the Greendale Fault near Darfield was produced after the 2010 earthquake.

Future GNS earthquake fault reports are scheduled over the next two years for Timaru, Kaikoūra, Waimate and Waitaki districts. Christchurch City has no known active fault lines at the ground surface.

For more information on the current report and others in the series, go to www.ecan.govt.nz/earthquakes

The Natural Environment Recovery Programme for greater Christchurch Whakaara Taiao, was launched recently. One of the 17 projects is to “Investigate and plan for natural hazards”. For information on the programme, go to www.ecan.govt.nz/nerp

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