‘Well behaved’ Alpine fault – experts respond

New research out today reveals that the Alpine Fault – a strike-slip fault running almost the entire length of the South Island – is surprisingly “well-behaved” in its regularity.

But good behaviour, in a scientific sense, may not bring much comfort to South Islanders.

In earthquake terms, the 850km-long fault is remarkably consistent, rupturing on average each 330 years, at intervals ranging from 140 years to 510 years.

The last major quake on the fault occurred 295 years ago, according to the GNS Science researchers who have compiled an 8000-year timeline of 24 major quakes on the fault from sediments at Hokuri Creek, near Lake McKerrow in north Fiordland.

Quoted in a press release for the paper, which is published today in the journal Science, the researchers including GNS principal scientist and manager of its Natural Hazards Research Platform Dr Kelvin Berryman, “calculate that there is a 30 percent chance a major earthquake will occur in Southern New Zealand sometime in the next 50 years”.

Long earthquake records have been obtained from only about four other faults around the world, including the San Andreas Fault in California. The latest findings underpin an idea over 100 years old – that some faults might rupture regularly, rather than at random or in clusters.

The scientists say: “We propose that it is valid to apply time-dependent earthquake recurrence models for seismic hazard estimation to similar faults worldwide”.

Other faults that have similar characteristics to the Alpine Fault include sections of the North Anatolian Fault in Turkey and the Denali Fault in Alaska.

The SMC gathered commentary from scientists on the new research:

Tim Stern Professor of Geophysics at SGEES and Institute of Geophysics
Victoria University, said:

” Berryman et al demonstrate an  impressive and compelling  geological record for periodicity  of displacement on  the southern Alpine fault at Hokuri Creek.  They also give an interesting  comparison between the displacement records of the San Andreas and the Dead Sea Fault, which  shows  all three faults behave in different ways.

“It would be interesting if a similar high-resolution record could  be found elsewhere on the Alpine fault, particularly near the central section of the fault. ”

Thomas Lumley, Professor of Biostatistics, University of Auckland

“The figure of 30% chance over the next 50 years looks plausible based on the data presented.  The intervals between quakes tend to be quite close to the average interval, with relatively little spread.

“Most of the recurrence intervals longer than 295 years (the position we are now in) are shorter than 400 years, and many of them are only slightly longer than 295 years.

“That is, most of the time when a quake hasn’t happened for 295 years, it happens within the next century and often within the next half-century. Working out an accurate probability would require more details of their model, but the conclusion does look reasonable.  

“The proportional variability in recurrence times around the average is much lower than in, say, the San Andreas fault in California (the authors say the Southern Alpine fault has a coefficient of variation of 0.3, and the San Andreas has 0.7).   This means risk of earthquakes can be predicted more accurately over timespans of decades in southern NZ than in California.

“Uncertainties in the model and the underlying earth movements will probably tend to lead to overestimates of the risk in this context, but based on the data, 30% in 50 years looks quite reasonable.

“Using the numbers in the paper I get 32% chance at 50 years, and 50% chance at about 80 years, both of which match the raw data fairly well.  The risks are high, but that’s because it seems to be an unusually regular fault.”

Mark Bebbington, Associate Professor at Institute for Information and Mathematical Sciences, Massey University:

“There are few implications to other known faults in New Zealand. The general case is that earthquakes can ‘load’ (bring closer to failure) some other faults, while ‘unloading’ (moving further from failure) other faults. It all depends on the orientations of the slips and faults concerned. As noted in the paper, the Alpine Fault is very unusual in that it does not get any significant loading from other faults, and can hence be examined in isolation.

Professor Richard  Norris, Department of Geology, University of Otago:

“This research is of international significance because such behaviour is often assumed in hazard calculations but rarely demonstrated. We must also point out that this record presented in the paper is for ruptures on the southern section of the fault. There is evidence that the northern section may rupture separately on some occasions, and that rupture frequency may be higher. The way in which ruptures break along the fault (i.e. their spatial distribution) is something that remains uncertain and needs further work. This paper however produces a superb record from the southernmost part.

“It doesn’t really change our understanding of the Alpine Fault, but it provides support for the view that the fault indeed ruptures at fairly regular intervals and has done so over several thousand years.

“The Alpine Fault has the highest level of probability for rupture of any fault in New Zealand. In comparison, the Greendale fault that broke in the first, September earthquake in Christchurch had a probability over a hundred times lower (and produced a much smaller earthquake)”

“Regarding areas at risk, Westland obviously is at high risk, with widespread damage likely and roads, bridges and other transport links likely to be badly affected (and the tourist trade). The fault crosses the west coast road in many places (e.g. just east of Haast, within Franz Josef township) and with an estimated 8 m displacement, will completely destroy it.

“It is important that the Westland Community is prepared for such an event and all possible mitigation options have been explored (e.g avoiding building on the fault where possible, using earthquake resistant building designs, avoiding areas likely to be hit by landslides, having a transport recovery programme in place, etc.).

“Intensities further east in places like Queenstown, Te Anau, Wanaka and Mt Cook will be high enough to cause landslips and do damage. Further east in the major cities of Christchurch and Dunedin, the intensities will be lower but the duration of shaking could still be sufficient to damage poorly constructed buildings (although Christchurch has already had most of those destroyed!) and possibly cause some liquefaction. Places such as Nelson, Wellington and Invercargill could also expect to feel some shaking. The intensity of shaking in Christchurch is likely to be less than that in the recent earthquakes although probably more prolonged”.