Canterbury Plains faults 'quite strong'

Last updated 11:07 21/09/2011
Mark Quigley
Mark Quigley says the Springs Rd area is being closely studied.
Jarg Pettinga
Jarg Pettinga says small fractures can grow and connect to others.

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Quake Science

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To what degree is the scatter of earthquake epicentres that we see away from the identified faults attributable to the highly deformed and (probably) broken up nature of the Torlesse group rocks? Is the Torlesse group basement behaving as a fragmented mass loosely adjusting through thousands of small (and scattered) movements to the major stress system changes caused by the main faults offsets? - John

The greywackes of the Torlesse group are indeed highly jointed and "broken up" at the surface. However, at the depths where crustal earthquakes occur (generally three to 12 kilometres or more in Canterbury), the highly deformed greywacke rocks will still have a complex rock fracture fabric, but the rock mass will mostly be tightly closed and not as "broken up" as it appears at the surface.

Nonetheless, aftershocks are initiating on such fractures and small faults at depth in the upper crust. Where the aftershock ruptures are able to propagate along such fractures, there is potential to grow and connect with other fractures, leading to larger-magnitude aftershocks.

Despite being situated in rocks that look pretty broken up at the surface, the faults beneath the Canterbury Plains are quite strong, probably because they have sufficient time to "heal" between major earthquakes. - Mark Quigley and Jarg Pettinga, Canterbury University

 

There have been a good number of quakes recorded at or near the Weedons Golf Club. This is just past the end of a documented faultline. Does the number suggest that the fault extends to the golf club site or further?

Also, there have been a large number of quakes in Springs Rd near Prebbleton, a long way from any shown faultline. Where does the large number of quakes in this area fit in the overall faultline picture and does it mean it is a quake-prone area? - A Douglas

The Weedons Golf Club is about 2.5km east of the easternmost mapped surface trace of the Greendale Fault that ruptured last September.

It is highly likely the Greendale Fault in some form continues eastwards for another few kilometres (but probably no more than 4km or 5km) in the subsurface beyond the mapped surface trace.

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Data obtained in the seismic reflection survey completed in May indicates a fault in the subsurface extending to near the intersection of State Highway 1 and Robinsons Rd, and this appears to align with a projected continuation of the Greendale Fault in the subsurface.

The Springs Rd area is earthquake-rich and has been the focus of ongoing seismological and geophysical investigations.

These studies indicate the presence of faults in the subsurface aligned northeast-southwest and extending from west of Halswell to west of Lincoln, as can be seen from the trends of aftershocks at GeoNet.

The seismic reflection surveys have imaged several faults in the subsurface in this area also. - Mark Quigley and Jarg Pettinga, Canterbury University

 

Professor Mark Quigley has stated that he expects, over time, for our seismic activity to subside as it appears to be doing at the moment - of course not over yet, but subsiding. That is heartening news. Unfortunately, the report ended with Professor Quigley stating there could be another earthquake of similar magnitude to September 4 within the next decade. My question is essentially why? And where in Canterbury would this be likely to happen? - Noel

Historical patterns of earthquakes around the world indicate that quakes tend to "cluster" in both time and space. Examples include both the Canterbury sequence and the Landers-Hector Mine earthquake sequence in southeast California in the 1990s.

These "triggered" earthquakes indicate that we cannot rule out the possibility that there could be another large quake triggered by the ongoing Canterbury sequence in the coming years to decades.

This is what Mark highlighted in his lecture, but he did not specifically say that this would happen in the next decade or even longer.

To be able to say exactly when and where this might occur would require us to know all faults capable of producing earthquake of this size, the state of stress on those faults before this sequence, the "breaking points" of these faults, and how the ongoing processes of stress transfer affect each of these potential earthquake sources. Scientific investigations into all of these points are ongoing.

Marine and land-based geophysical surveys in eastern Canterbury and offshore have identified fault zones of sufficient length to generate large earthquakes but the recurrence intervals of earthquakes on these faults are very long (many 1000s to 10,000s of years and longer).

Statistical methods estimate a 2 per cent probability of an earthquake of the magnitude of the September event happening in the next year.

What is sure is that New Zealand will experience large earthquakes in the future and, on average, they will be in the main seismic belt that trends diagonally through the country from northeast North Island to southwest South Island. - Mark Quigley (Canterbury University) and Matthew Gerstenberger (GNS Science)

I am wondering if GNS is any closer to constructing a map that clearly shows where the faults lie at street level? I know this question was asked a couple of months back and the answer was that they were working on it. I know a lot of people would be interested in seeing such a clear map. - Gillian

Maps of the Greendale Fault that ruptured the surface in the September earthquake are widely available through the GeoNet active faults database and in research publications available by clicking here.

The average width of the fault zone is 80 metres at the surface, and in some places the deformation zone is over 1km wide.

Many of the faults in the Christchurch region, including those that ruptured in the February and June quakes, did not rupture through to the surface. So a map of where these broad "zones" of buried faults, commonly several hundred metres wide, might project to a certain street address would be hampered with uncertainty and thus be somewhat irrelevant.

Approximate locations of faults in the subsurface can be viewed as part of the Canterbury University earthquake lecture series available by clicking here. - Mark Quigley and Jarg Pettinga, Canterbury University

* Email your questions to Vicki.Anderson@press.co.nz.

- © Fairfax NZ News

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