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Drilling deep into the Alpine Fault

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Kiwithrottlejockey
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« on: April 14, 2012, 01:28:12 pm »


Over-optimistic to a fault

When will there be an earthquake on the Alpine Fault? Tim Davies, Jarg Pettinga,
Carolyn Boulton, Greg de Pascale and Tom Robinson have been reviewing the data.


The Press | 11:53AM - Friday, 13 April 2012

ZEALANDIA CONTINENT.

ON APRIL O2, The Press reported that the threat of an Alpine Fault earthquake has decreased, based on data from trenches recently excavated across the fault south of Haast.

Instead of the previously accepted 200-to-300-year interval between these major earthquakes (M equals 8 or more, about 30 times the energy of the September 04, 2010, earthquake), the trenching shows that the interval is about 50 per cent longer; 485 years.

Since the most recent earthquake on the Alpine Fault occurred about 1717 (295 years ago, very close to the "old" recurrence interval), the implication is that we can all relax and stop worrying about the fault, because it might not cause a major earthquake for another 200 years.

It has become clear since September 04, 2010, that people want the best information they can get about earthquakes, so this good news should be welcomed widely.

However, initial responses from the public have been largely sceptical, and correspond to our own conclusions that the report is over-optimistic.

Scientists and the media need to provide complete and accurate information to the public but, in this case, the information appears to have been presented in an overly positive light, for the following reasons:

First, it applies information from trenches within 12 kilometres of Haast to the entire Alpine Fault.

The fault extends more than 600km from Milford Sound in the south to Blenheim in the north, so data from 2 per cent of this length may in fact tell us little about most of the fault's behaviour.

Previous individual Alpine Fault earthquakes have ruptured lengths no longer than about 400km, so may not extend to the south of Haast or be recorded there.

For example, there are independent records of large earthquakes north of Haast in 1717, circa 1625, circa 1425 circa 1300, circa 900, and circa 400 (as documented by Berryman et al in 2009) with a recurrence interval of about 300 years.

At Hokuri Creek, 100km south of Haast, 8000 years of records give a recurrence interval of about 320 years, according to a paper by Biasi, Clark, Berryman, Cochran and Prior presented to the American Geophysical Union in 2010.

The 485-year return interval from the Haast trench data evidently does not apply north or south of the site.

Second, the long-term tectonic slip rate (the rate at which the Pacific and Australian plates are sliding past one another), as calculated from the Haast trench data, is about 17 millimetres a year, about 50 per cent less than that on the rest of the fault.

On the central Alpine Fault between Haast and Hokitika, slip rates are 27mm, plus or minus 5mm a year (Norris & Cooper, 2001); in the Cascade area, 50km south of Haast, they are 23mm, plus or minus 2mm a year (Sutherland, Berryman and Norris, 2006); and, 80km further south at Milford Sound, the slip rate is about 26mm a year (Barnes, 2009).

The slip rate deficit from the Haast trenches suggests that some earthquakes may not have been recorded or identified there.

Thus, the average return interval of 485 years may apply to the limited area of the trenching, but there is some doubt about that; and there is strong evidence that it does not apply to the 300km of the Alpine Fault north of Haast.

The evidence that the threat of a major alpine earthquake anywhere on the Alpine Fault has receded is, in fact, very weak.

In any case, the question of overriding importance to the population and commerce of the South Island is not the average return period between earthquakes, but when the next major earthquake will occur.

It will occur at some unknown (but fixed) time in the future; the data referred to above indicate that it could happen today, or after some centuries, or at any moment in between.

Therefore, the most rational approach to the next major earthquake is to accept that it can occur at any time, and make preparations as soon as possible.

These range from stocking up with food and other supplies, all the way to reorganising society and commerce to be less vulnerable to the earthquake and its consequences, and thus better able to redevelop afterwards.

Some of these preparations can (and should) start immediately, but many will require gradual implementation; however, the sooner they begin, the less damaging the earthquake will be.

It is also vital to recognise that any Alpine Fault earthquake and its aftershocks will cause substantial disruption to the landscape.

Current research at the University of Canterbury shows there will be many large and small landslides, likely causing tsunami in lakes and fiords, landslide dams and dambreak floods in rivers, widespread aggradation and avulsion of rivers, and debris flows in small creeks, as a result of massive sediment inputs. These effects may continue to disrupt recovery efforts in alpine and adjacent regions for years to decades after the Alpine Fault earthquake.

This research into the magnitude of the earthquake and its effects is ongoing.

While the "threat" of an Alpine Fault earthquake may appear to have waned, that earthquake will still occur when it would have occurred anyway, irrespective of attempts to refine the statistical parameters of the fault.

It would be a serious mistake for authorities or public to breathe sighs of relief, in the belief that disaster has been postponed — it hasn't.


  • Tim Davies and Jarg Pettinga are professors in the Department of Geological Sciences, University of Canterbury;
    Davies leads the University's Hazard and Disaster Management programmes. Carolyn Boulton, Greg de Pascale
    and Tom Robinson are doctoral researchers at the University of Canterbury.

http://www.stuff.co.nz/the-press/opinion/perspective/6735254/Over-optimistic-to-a-fault
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