Quake research focus on Rangiora, Oxford

03:44, Nov 29 2013

An earthquake faultline which passes directly through Oxford is the most active in the region, but scientists say its existence is not yet proven.

Environment Canterbury (ECan) this afternoon released its latest in a series of earthquake fault reports.

The Selwyn and Waimakairi areas were studied by GNS Science, mainly because there had been rapid population growth in those areas.

More detailed reports are available on ECan's website.

It identifies possible faultlines near both Rangiora and Oxford though scientists say more research is needed on both.

Suspected fault near Oxford yet to be confirmed


The Starvation Hill fault and Ashley Fault Zone are the only active fault/fold features lying beneath, or close to, towns or settlements.

The report states: "In particular, it would be desirable to confirm whether or not the Starvation Hill fault, which passes through Oxford township but whose existence is suspected rather than proven, is in fact an active fault, and if so whether it is sufficiently active to warrant hazard avoidance measures."

The recurrence interval estimates indicate that the Lees Valley Fault, the Knowles Top fault zone, the Starvation Hill fault and the Ashley Fault Zone are the most active faults in the district.

The remaining faults or folds appear to have a "relatively long recurrence interval".

Detailed report due on Rangiora fault

Scientists are also studying the Ashley-Loburn fault, north of Rangiora with a detailed report due to be released soon.

The Selwyn report states that the Poulter Fault in the northwestern part of the district and Porters Pass Fault in the Southern Alps foothills, were the most active faults in the Selwyn District.

"Of the other faults or folds in the district each appears to have a relatively long recurrence interval, but collectively there are many of them."

In total, there were 24 areas of known or suspected active faults and/or folds.

In the Waimakariri report, 15 of known or suspected active faults and/or folds are outlined.

The report said while there were several active faults in both districts, the probability of a fault rupturing was low.

"In a year, there is a one per cent or less chance of a fault rupturing somewhere within the district. And for residential areas, it is 10 times less likely again,'' the report said.

Most faults found were found in sparsely populated areas.

In Selwyn, two towns - Sheffield and Hororata had "likely'' faults nearby which were yet to be confirmed.

It said there were no known faults near Lincoln or Rolleston.

Most Selwyn faults had a recurrance interval of several thousand years. Only three were found to have shorter intervals: Porters Pass, Harper and Acheron faults in the western part of Selwyn.

In Waimakariri, both Rangiora and Oxford had "likely'' faults nearby, though these were yet to be confirmed. It said there were no known faults in Kaiapoi.

Again, like Selwyn, the faults were expected to have an interval of several thousand years between rupturing.

GNS plans further fault studies over the next two years for Timaru, Kaikoura, Waimate and Waitaki districts.

Christchurch City has no known active fault lines at the ground surface.

New quake maps will be used by councils

ECan commissioner Donald Couch said the Selwyn and Waimakariri districts were prioritised for fault research because of the unprecedented growth in population since the 2010 and 2011 earthquake. "It is therefore important that we understand the risk profile."

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 affect a much wider area.  

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

Couch said ECan and Selwyn and Waimakariri district councils would use the report for land information requests (LIRs), land information memoranda (LIMs), public education, and planning and policy.


The Press