Rock fracturing causing volcano quakes

MICHAEL DALY
Last updated 14:54 25/01/2013
White Island
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Activity on White Island captured by a GNS camera.

VOLCANO: White Island.
ACTIVE: The volcano at White Island.

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Hybrid earthquakes recorded at White Island in recent days are thought to be the result of some kind of rock fracturing, and are often attributed to magma movement within a volcanic system, a scientist said today.

"The really high levels of tremor that we've seen over the last couple of weeks, and then coupled with these new hybrid earthquakes means that our concern right now is quite high," GNS Science head of volcanology Dr Gill Jolly told a briefing for journalists.

Yesterday afternoon the aviation colour code at White Island was raised to orange, the third-highest of four codes. It indicates heightened unrest at the volcano with increased likelihood of an eruption.

Jolly warned that visitors to the island were now at the highest level of risk since the start of eruptions last August.

Hazards could include the health effects of volcanic gas exposure, including respiratory issues, and skin and eye sensitivity to acidic gases.

"Explosive eruptions can occur at any time with little or no warning," she said.

"We advise a high level of caution should be taken, if visiting the island."

Jolly said hybrid earthquakes were so-called because they had a mixture of different frequencies, with very high frequency at the start – a kind of impulsive start – and a kind of wobble afterwards.

"It's a bit like striking a bell," she said.

"The initial impulse is high frequency and then you have the resonance of the bell."

It was hard to tell what might happen in the next few weeks, but it was possible White Island could become more active if a significant batch of new magma came up through the volcano.

Geysers at a crater lake could get more vigorous, and there could be more volcanic explosive activity such as the venting which happened in August, or it could perhaps be even more vigorous.

Between 1976 and 2000 some of the larger explosions ejected blocks of magma on to the crater floor, with some even reaching the sea.

"But with all volcanoes there is a lot of uncertainty, a bit like trying to forecast the weather with your eyes closed because you can't actually specifically see what's happening under the surface," Jolly said.

"They often look like they're building up to something and then nothing happens at all."

If the amount of gas measured over the volcano went up it could indicate more magma was coming up into the system.

If there were a lot more earthquakes it would suggest something was pushing from below, probably magma being forced into the near subsurface in the volcano. More heat in the system could also indicate a new batch of magma coming close to the surface.

White Island had been active between 1976 and 2000, with regular ash venting, small explosions, and larger explosions throwing blocks out about every three to four months on average, Jolly said. Since 2000 it had been relatively quiet, with no surface activity.

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The most-recent episode started last July, when scientists were alerted by a rapid change in the lake at the crater, with the level moving by five to seven metres over one night.

Then in early August there were some weak eruptions of ash, which was found to be new magma. It was the first ash erupted on White Island since 2001.

A mass of material was extruded producing a lava dome, probably in late November.

Jolly said no huge changes had been found in regular gas measurements at the volcano, suggesting that even if there was new magma the volume was not large.

Then in late December and early January White Island appeared to be quiet.

"We actually sat down and met in here on January 14 and were reasonably comfortable that the hazard had decreased," she said.

"Within 24 hours of meeting the volcano decided to do something different and it started to ramp up."

Volcanic tremor, a vibration in the volcanic system, increased and small explosions were seen in the remains of the lake at the crater.

"Now, we know there's magma very close to the surface because we have the fresh magma erupted in August and then the dome extruded in November," Jolly said.

"What we think is actually happening is that the dome extruded and that's almost like a plug on that part of the hydrothermal system. So there may well be heat still coming up through with the magma.

"That's effectively expanding the steam zone around the dome, and that might be what's increasing the tremor and producing this geysering at the lakelet which is some distance away from the dome."

Nearly all eruptions on White Island only affected the island itself. Rocks had been ejected up to a few hundred metres offshore in the past, she said.

Depending on the wind direction, a light ashfall was possible on the mainland in a larger scale eruption, but it was unlikely to have a big impact.

- Stuff

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