Hurunui earthquake fault report released

Environment Canterbury today released the third in a series of earthquake fault reports on different areas of the Canterbury region.

Environment Canterbury Commissioner Donald Couch says the Hurunui District is a priority location for a fault survey. “This area has more active faults than any other part of Canterbury,” Mr Couch said. “Historically it has had more earthquakes than any other area.”

However, most of the faults and folds identified on the ground surface in Hurunui District 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 district.

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 horizontally 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 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.”

Following the Ministry for the Environment Active Fault Guidelines, normal residential development would be allowed on or near faults with long recurrence intervals. Planning restrictions may apply to larger community buildings.

Mr Couch says Environment Canterbury and Hurunui District Council will use the report for land information requests (LIRs), 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.

A Hurunui District lifeline project aims to identify vulnerability of engineering lifelines and emergency services to damage from natural hazards such as earthquakes, and to identify practical strategies to reduce their risk and impact.

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.

If appropriate, the reports highlight “fault avoidance zones” where government guidelines recommend that development may not be permitted, or where only certain types of buildings may be constructed.

Earlier reports covering the Ashburton and Mackenzie districts, the latter including detailed fault avoidance zoning of the Ostler Fault at Twizel, were released in 2009 and 2010 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 three years for Selwyn, Waimakariri, Timaru, Kaikoūra, Waimate and Waitaki districts. Christchurch City has no known active fault lines at the ground surface.