Volcanic cracks shed light on potential eruptions

New research has disclosed telltale signals which would have enabled New Zealand geologists to warn people of the 1995 Mt Ruapehu eruption a year before it happened.

Geophysicist Alex Gerst, of Victoria University, and colleague Martha Savage say seismometers installed around Mt Ruapehu showed that some parts of seismic waves from quakes changed in the way they behaved between 1994 and 1998.

If geologists had been aware that the changes signalled an impending eruption, they could have warned of Mt Ruapehu's eruption a year in advance, the New Scientist magazine reported.

Better warnings of eruptions could have implications for regional economies in the North Island - the 1995 and 1996 eruptions led to many tourists and skiers taking their recreational spending elsewhere, with particular impact on the Ohakune district.

The eruptions were the volcano's largest in modern history and occurred with little apparent warning. Other major eruptions in 1945, 1969, 1975, 1981 and 1988 also occurred with little warning.

Other academics, such as Professor David Lowe, of Waikato University, have previously called for volcanic hazard assessments to consider the effects of "distant" eruptions after the threat of ash from the ongoing Ruapehu flare-up in 1996 closed Auckland Airport for three nights.

The closure of airports, respiratory problems, and the disruption of electronic systems were highlighted by a $300,000 report by Professor Lowe, which found that ash from an eruption in the central North Island could harm Auckland.

The Waikato regional council estimated in 1999 that although the Mt Ruapehu eruptions in 1995 and 1996 caused only minor damage and disruption, the financial losses totalled at least $130 million.

This included a big insurance company which had to pay $4.6 million to operators of the Whakapapa ski field for business lost through snow damage on Mt Ruapehu.

Dr Gerst said the hard rock around volcanoes such as Mt Ruapehu contained millions of tiny fluid-filled cracks that run parallel to the edges of the blister-shaped chamber of magma beneath the volcano.

Earthquakes many kilometres below the surface constantly created seismic waves.

One type of seismic waves - the S-waves which deliver most of a quake's shock - do not travel through liquids, so their direction of travel under the volcano is influenced by the alignment of the fluid-filled cracks in the rock.

In effect, travel through the earth's crust under the volcano separated the S-waves into "fast" and "slow" components, similar to the way polarised sunglasses filter out light rays not aligned with the lens.

Dr Gerst said that as the magma built up below the volcano, it pushed hard against the surrounding rock, causing the cracks parallel to the magma chamber to close.

At the same time, new fluid-filled cracks perpendicular to the chamber open up - which significantly affected seismic waves.

"It's like running through a field filled with parallel streams," said Dr Gerst. "If you run parallel and never cross a stream, you will be faster than if you run perpendicular and have to spend time hopping over the streams."

The S-wave seismic waves flipped their orientation by up to 90 degrees between 1994 - the orientation before the eruption - and 1998, which the scientists believed represented the normal state for the volcano. New readings taken in 2002 showed no change compared with 1998. The area covered by the seismometers was about 100sq km.

"The exciting thing is we can measure the stress of a volcano ... it lets you look through the volcano like an x-ray," Dr Gerst said.

Once the delay between changes in the direction of the cracks and the subsequent eruption had been pinned down, or the existence established of a "stress threshold" for an eruption, predictions could be made for new eruptions.

"Achievable warning times could be months to a few years in advance," Dr Gerst said. The changes measured between 1998 and 2002 suggested the technique could be also used to monitor more subtle real-time stress changes in and around large magma chambers, short of a large eruption.

The finding could also apply to volcanoes with thick viscous magma, such as Mt Etna in Sicily and Mt St Helens, in Washington state.

However, Dr Gerst said while Ruapehu's magma build-up was strong enough to overcome local stresses that kept the cracks open in one direction, this might not be the case elsewhere - for instance at the volcanoes in Hawaii which had runny magma.

Steve Malone, a geophysicist at the University of Washington in Seattle, who studies Mt St Helens, said the tool could fill a gap in forecasting, which was very long-term or very short-term at present.

"It's an exciting development," he said. "If it turns out not just to be serendipitous, it could also be very powerful."

ERUPTION LOSSES

Financial losses from the 1995 and 1996 Mt Ruapehu eruptions were estimated at $130 million.

They included a $4.6 million insurance payout for lost business on the Whakapapa skifield.

Other problems were airport closures, respiratory problems, and the disruption of electronic systems.

- NZPA