Preparing for the big one

A great deal of good work has been done in the past few years to bring South Islanders up-to-date with the chances of a large earthquake occurring and how best they can prepare.

The South has many potential sources of damaging earthquakes. Recently, the spotlight has been turned on some of the geological faults which run close to Dunedin and the southeastern part of Otago.

However, the feature which remains of most concern to the greatest number of people is the Alpine Fault — a highly visible gash more than 800 kilometres long on the western side of the Southern Alps, which marks where the Australian and Pacific crustal plates are graunching against each other.

Scientists have determined that the last major earthquake generated by this fault was in 1717 and of magnitude 8.1. Using a variety of methods, they have calculated the Alpine Fault wakes from its slumber roughly every 300 years, which means another big earthquake is about due.

Science communicators have been busy talking to South Island communities about how this event will affect them and what they can do to be ready for it.

Much of this has been done by team members of the extremely valuable AF8 project. This began about five years ago as a collaboration between universities, crown research institutes and others, and marked a major shift in direction to more closely involve the non-scientific public of New Zealand in ongoing research.

As the AF8 roadshow has continued rolling out, scientists have been carrying on with crucial studies of the fault, with each set of findings revealing fascinating, but worrying, pieces of the Alpine Fault puzzle.

Until last week, the experts’ best estimate was there was about a 30% chance the fault would generate a major earthquake in the next 47 years. However, new research led by Victoria University of Wellington and former University of Otago scientist Dr Jamie Howarth shows the risk is more likely 75% by 2068.

The researchers also found, somewhat alarmingly, there is an 82% chance the next Alpine Fault earthquake will be of magnitude 8 or higher.

While the fault looks razor sharp from space, on the ground it is split up into segments which have slightly different orientation.

One of the team’s most interesting findings is of some kind of rock structure on the fault at Martyr River, southwest of Jackson Bay, between segments. This “gate” either stops, or lets through, ruptures during an earthquake.

When this “gate” halts a rupture, those earthquakes are large, of magnitude 7 or so, roughly equal in power and energy to the September 2010 Darfield earthquake. But when it lets it through, it can grow into a great earthquake of magnitude 8 and above.

Sediments retrieved from four lakes and two swamps on the West Coast revealed 20 Alpine Fault earthquakes in the past 4000 years. They showed the past three events went through the gate and generated great quakes.

Using modelling, the researchers deduced the next one is also likely to pass through the gate and be similar to the 1717 event.

These results are certainly a wake-up call for South Islanders, particularly residents of the West Coast and those living in western Otago in Queenstown and Wanaka.

The leap from a 30% to 75% chance in the next 47 years is significant and potentially alarming. But rather than be overly anxious about the figures, we should let them provide impetus for our preparations for the big one.

This latest research represents another step on the journey to gaining a more accurate understanding of the Alpine Fault. Scientists have come a long way since a wall was built across the fault near Springs Junction in 1964 to judge whether the fault was creeping or was locked. Fifty-seven years later the wall has yet to move.

We should remember the findings have not actually made the Alpine Fault any more dangerous, or hazardous, than it already was. Instead, we are just becoming better educated about it.

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