CHANGES in the direction of seismic waves around active volcanoes could help warn of an impending eruption almost a year before it happens. New Zealand researchers say they have identified a telltale signature for Mount Ruapehu, which blew its top in 1995, and the finding could apply to other volcanoes.
The hard rock around volcanoes such as Mount Ruapehu contains millions of tiny cracks that run parallel to the edges of the blister-shaped chamber of magma beneath the volcano. Earthquakes tens of kilometres below the surface constantly create seismic waves, which the cracks separate into fast and slow components. “It’s like running through a field filled with parallel streams,” says geophysicist Alex Gerst of the Victoria University of Wellington in New Zealand. “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.”
Seismometers installed around Mount Ruapehu showed that the fast components of seismic waves flipped their orientation by up to 90 degrees between 1994 and 1998. The 1994 readings represent the orientation prior to the eruption, while those from 1998 represent the normal state for the volcano. Readings taken in 2002 show no change compared with 1998 (Science, vol 306, p 1543). Any changes in the direction of the fast components signal an impending eruption, say Gerst and colleague Martha Savage. If they had known this in 1994, they could have warned of Mount Ruapehu’s eruption a year in advance.
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But what causes the waves to change direction? As the magma builds up below the volcano, it pushes hard against the surrounding rock, causing the cracks parallel to the magma chamber to close up, while opening new cracks perpendicular to the chamber. This has a significant effect on the seismic waves. “The exciting thing is we can measure the stress of a volcano in situ,” says Gerst. “It lets you look through the volcano like an X-ray.”
The technique could be used to monitor Mount Etna in Sicily, Mount St Helens in Washington state and Mount Spurr in Alaska, among others, because they are similar volcanoes with thick viscous magma, unlike the volcanoes in Hawaii, which have very runny magma. However, Gerst says that while Ruapehu’s magma build-up was strong enough to overcome the local stresses that keep the cracks open in one direction, this may not be the case elsewhere.
Steve Malone, a geophysicist at the University of Washington in Seattle who studies Mount St Helens, says the tool could fill a gap in forecasting, which is very long-term or very short-term at present. “It’s an exciting development,” he says. “If it turns out not just to be serendipitous, it could also be very powerful.”