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When you can't see the volcano for the clouds

9 January 2013

Infrasound monitoring detects volcanic eruptions and contributes to public warning system.

By Rachel Berkowitz

Forecasting natural disasters has featured in the news lately. In the wake of the 2009 L'Aquila earthquake, six Italian scientists and one government official were sentenced for manslaughter for providing 'falsely reassuring' information about an event that proved devastating.

The American Geophysical Union issued an official statement that the Italian court's verdict was 'troubling and could ultimately be harmful to international efforts to understand natural disasters and mitigate associated risk.'

'Understand[ing] natural disasters and mitigat[ing] risk' is a vital goal for the US Geological Survey. The Alaska Volcano Observatory (AVO) conducts scientific investigations in order to assess the nature and likelihood of volcanic activity, to assess volcanic hazards associated with anticipated activity, and to provide timely and accurate information about and warnings of impending dangerous activity.

AVO goes about this mission in several ways. Some volcanoes in Alaska—those relatively easy to access—have seismic networks installed nearby. Network data becomes a monitoring tool to detect and forecast volcanic activity. 'Seismic signals can give us information about what is going on beneath the volcano, often before there is any activity at the surface, like explosions,' explains AVO vulcanologist Kristi Wallace.

But because not every volcano has its own seismic network, remote sensing satellite techniques are also used to track a volcano's activity. But this does not always work, as in the event of cloudy weather. And, since imaging provides information about events that have already occurred, it is more useful for monitoring than for forecasting.

This is where infrasound comes in to play. Infrasound refers to acoustic waves generated at lower than 20 Hz, and the infrasound generated by volcanic explosions can propagate over thousands of kilometers.

In a study published in Geophysical Research Letters in October, Silvio De Angelis and colleagues at the AVO report on twenty explosions at Mount Cleveland, a 'moderate' threat stratovolcano in Alaska's Aleutian Island chain; the explosions were detected by seismic and infrasound observations in the first half of 2012. Mount Cleveland is primarily monitored by satellites, and the closest seismic network is located 75 km away.

'The use of infrasound has been a great new tool because it can allow rapid detection of explosions at volcanoes...if there are nearby instruments [such as those on adjacent islands],' says Wallace.

Infrasound sensors across Alaska and distal seismic instruments have already recorded activity at the remote Mount Cleveland. Signals associated with eruptive activity register at seismometers in other areas, in the form of ground-couple acoustic signals. Acoustic waves from the volcano load Earth's surface from the atmospheric pressure field associated with the acoustic wave field, and these slow-velocity waves are recorded by seismometers.

In the De Angelis study, the seismic network on Okmok volcano 120 km away from Mount Cleveland was fitted with a scanner to determine whether detected signals exhibited the expected time delays for an airwave from Mount Cleveland. Models traced infrasonic propagation across the Aleutian arc, a useful exercise for determining future instalments of infrasound detectors. Those detectors would provide a direct measurement of the acoustic wavefield, rather than relying on secondary air-to-ground coupling detected by seismic instruments.

The seismic system is set to warn AVO scientists so that they can issue information statements and notify the public of volcanic activity. Since infrasound is still a detection tool and not a forecasting tool, it is possible to miss signals that may be indicative of further activity. But in Alaska, where many active volcanoes exist without ground-based instrumentation, establishing a distant monitoring system is critical.

If possible volcanic activity is detected, the Scientist-In-Charge and Coordinating Scientists of the AVO notify critical agencies. The National Weather Service provides the Alaska aviation community, other government agencies, and the public with forecasts and warnings.

While the implications of warnings produced by a US volcano observatory are different from those produced by Italy's Commission for the Forecast and Prevention of Major Risks, the importance of developing the best possible techniques and implementations for assessing natural hazards is clear.

USGS spokesperson Justin Pressfield says regarding the L'Aquila earthquake that 'as a federal science agency, it would be inappropriate for the USGS to comment on the legal proceedings and outcome of the Italian courts.' But these verdicts, which shocked the global geological community, may promote renewed commitment on the part of scientists and government officials to strive for the best possible measures to protect the public.

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