Feature

Volcano Alert

New Zealand's volcanoes could erupt at any time, but will we have any warning?

By Vicki Hyde, NZSM

A couple of earthquakes hit the Bay of Plenty. Nothing unusual perhaps, except that these quakes mark the start of renewed volcanic activity in the ancient, dangerous Okataina volcanic area east of Rotorua.

New steam vents open up and temperatures in the local streams start to rise. Geologists and geophysicists descend on the area, racing to put seismographs and monitoring equipment in place to give them a chance to assess the new activity.

One month after the first earthquakes are noted, a new vent erupts near Haroharo lava dome, throwing up 1,000 million cubic metres of ash which settles over 15,000 square kilometres. The eastern half of the central North Island disappears under a fine grey powder, 17 people are killed by ash and mud slides, and hundreds are left homeless.

It's all in the minds of Civil Defence as part of their focus on volcano hazard awareness, but it is something which might really happen at any time. The major question is, will we have enough warning for any preparation?

Perhaps not, according to Dr Paul Froggatt of Victoria University. Like many geologists working on volcanoes, Froggatt is concerned at the lack of long-term monitoring being done in New Zealand's numerous volcanic areas.

Seismometer Struggles

"Each volcano needs a minimum of three, preferably five, seismometers," Froggatt contends, noting that most areas have only one or two, if any.

The seismometers can record earthquakes associated with volcanic activity, and distinguish them from the more usual tectonic earthquakes. By placing them around the volcanic area, volcanologists can get some idea of the depth of the earthquakes and the potential strength of the activity. Extra stations are useful for cross-checking data in the event of breakdown, high winds or other problems.

The volcanologists have to vie with seismologists for the placement of seismometers. Dr John Latter, of DSIR Geology and Geophysics, has seen some competition for the limited resources.

"You try and persuade the seismic people to get them close to the volcanos, but the seismologists want them as far away as possible from the source of noise," he notes.

Volcanically-triggered earthquakes are shallower and more localised than normal tectonic ones. Place a seismograph too far away from a volcano and you can't pick up the former. In particular, you cannot observe the continuous volcanic tremor, the harmonic tremor, which is often a sign of an impending eruption. Place a seismograph too close to a volcano and it becomes difficult to pick up clean signals from distant, regional earthquakes.

For many years, the nearest seismograph to Mt Egmont was 30 km away at Tarata.

"You don't see volcanic tremor at that distance," says Latter. "If you do, it probably means there's an eruption in progress."

In 1985, a station was placed four kilometres from the summit of the dormant volcano, but geologists recognise that Taranaki still represents a major hole in the country's monitoring network. There are plans for a joint network to be developed by DSIR and Auckland and Massey Universities, operated in cooperation with Civil Defence. Auckland University geologist Dr Ian Smith hopes to see the first seismometer in place by Christmas, with five more to follow by the end of next year.

While the universities are interested in the network's potential as a research tool, they are aware of the need for continual monitoring.

"It might have to go in for the long haul, a hundred years or more" says Smith.

Long-Term Monitoring Needed

Froggatt is keen to see a comprehensive monitoring network put in place to provide long-term data. He estimates a cost of around $50,000 per volcano to establish an automated monitoring system that would provide a useful database on the state of the country's volcanoes. Given the potential for billions of dollars worth of damage in the event of an eruption, such a system would be cheap.

"It's like an insurance policy -- most people begrudge paying policies but, on the rare occasions they need them, they're thankful," maintains Froggatt.

Funding such a system would be a major exercise in itself. Volcano monitoring is concerned with very long time scales, with hundreds or thousands of years between major eruptions. Hence, monitoring programmes have to be sustained virtually indefinitely. With much of science already short of cash, it's unlikely that extra funding will be allocated to the equipment and staff needed. That doesn't stop volcanologists from hoping for the establishment of at least a minimum effective monitoring system.

"Nature is giving you a long time that, if you're sensible, you can make use of," notes Latter. Monitoring during quiet periods gives volcanologists a baseline from which to work. It also provides an opportunity to put various preventive measures in place, such as earthworks to deflect lahar mud flows from populated areas.

"You need long-term, on-going monitoring so you can decide what is normal for the volcano," adds Froggatt.

The Auckland field, for example, is very quiet, geologically speaking. The onset of minor earthquakes there could well herald a new phase of volcanic activity. Should such activity arise, geologists and geophysicists would be quickly on the spot, albeit with limited resources and possibly too late.

"We could probably scrape up enough instruments to mount a local operation," Latter says. "The danger is, sometimes these things can start very quickly. You have to have your stuff in and ready during the quiet times to get it calibrated and working properly."

Danger Zones

Latter is well aware of the likelihood that Auckland will one day cause a major problem. The area is dormant, not extinct, and although the activity is likely to be small-scale, even a "small-scale" eruption could be catastrophic, occurring as it would within a major metropolitan area. The region has almost 50 volcanic cones, many no longer visible, and it has been only a few hundred years since Rangitoto last erupted.

The area which really has volcanologists worried is Taranaki, with Mt Egmont consistently identified as one of the most dangerous volcanoes in the country. While dormant at present, the mountain has the potential for providing a major disaster.

"Taranaki bothers me because it has a very active history, erupting every 320 years -- it last erupted 250 years ago. At the moment, it's got virtually no monitoring, it's very dangerous and it's surrounded by Taranaki farms and towns," says Smith.

"Egmont is very dangerous because, although it doesn't have very large eruptions compared to those of the Taupo-Rotorua area, they do lead to collapse of the cone quite often," says Latter.

This causes major debris avalanches to sweep over the ring plain, and devastating lahars to pour down the valleys towards the coast. It's happened at least three times in the past, releasing more than 3.5 km³ of material across 40 km. The Taranaki province will not be the only area to suffer should Egmont erupt -- ash from past eruptions has been found as far afield as Hamilton and Napier.

Ruapehu has caused considerable damage from lahars in the past, and deaths resulted from one in the Tangiwai disaster of 1953. A mud flow destroyed the Tangiwai Railway Bridge shortly before the Wellington-Auckland express was due to cross, leading to the loss of 151 lives. There are now monitors in place to warn of lahars, and skiers at the Whakapapa skifield have a 10-minute alert in the event of an emergency.

Taupo is another very dangerous area, having been the site of huge eruptions in the past. Some 2,000 years ago, an enormous eruption took place, forming a major part of the huge basin now occupied by Lake Taupo. Pumice was blasted as far away as Hawkes Bay and ash deposits reached depths of as much as 70 metres.

"There's not an awful lot you can do to get away from that," remarks Latter dryly. Froggatt has found that people in the Taupo area tend to pick the Taupo-Napier highway as a potential escape route from any volcanic activity.

"That road goes down the dispersal direction. Every eruption there for the last 40,000 years has dumped ash on Napier," he notes.

A route north to Rotorua would be the safest, despite the apparent danger of moving further into a volcanically active area. The two fields are not connected in a direct way, and an eruption at Taupo is unlikely to set one off at Rotorua.

Playing The Odds

The scientists are aware of the potential problems of providing volcano alerts. Overseas, lives have been lost while geologists, politicians and civil authorities wrangled over how to interpret data.

In 1902, evacuation of the town of St Pierre, threatened by Mt Pelée, was halted by the governor so that an election could be carried out. The eruption took place three days before the election, killing more than 28,000 people. Two people survived.

"In New Zealand, the Civil Defence organisation is confident and competent enough that that wouldn't happen," Froggatt says.

Appropriately enough, this year is Nga Puia, the year of looking at contingency plans for volcanic eruptions. Civil Defence's Volcanic Hazards Working Group is in the process of publishing a series of comprehensive booklets on the various volcanoes within New Zealand's territory. While volcanic eruptions tend to be rare events, Civil Defence are aware of the need to have appropriate preparations in place.

With volcanoes, people tend to play the averages game, notes Froggatt. They assume that, because a volcano has an average return period of 500 years and last erupted 200 years ago, it won't do so again for another 300 years. Such reasoning is specious and dangerous.

"Averages mean that some lengths of time are short," he says.

Froggatt recalls the burst of funding poured into earthquake research following the large Inangahua earthquake and wonders if any signs of volcanic activity would spark an interest in monitoring. He remains largely pessimistic.

"There's an old adage that the next disaster will strike just as the previous one is lost to memory."

Vicki Hyde is the editor of New Zealand Science Monthly.