Friday, 27 January 2017

Volcano monitoring from a distance

In the past few weeks, there has been an eruption that keeps littering my inbox with emails: Bogoslof Volcano, on a tiny island of roughly 1 by 2 km out in the Bering Sea, west of the Alaska Peninsula.

View from a helicopter onto Bogoslof Island. Photo: Dan Leary, Maritime Helicopters
Despite the fact that it's effectively in the middle of nowhere (the nearest town is roughly 100 km away), Bogoslof is an interesting one. Being up in the Aleutian Chain, it sits along a very important corridor for international air traffic. If you remember the chaos all over Europe after the 2010 eruption of Eyjafjallajökull in Iceland, it's hardly surprising that monitoring volcanoes even in parts of the world as remote as Alaska is an important task. But how do you monitor a volcano that sits on an uninhabited, far away island?

An obvious answer would be to put a bunch of instruments onto the island. However, the island is so small, so far away from any population, in such a harsh environment, that the Alaska Volcano Observatory has to focus its limited resources elsewhere. In addition, the last eruption previous to this one had occurred in 1992, and it's been at least 40 years since the last eruption before that, so unsurprisingly the volcano was relatively low on the monitoring priority list.

This changed on 20th December 2016, when several pilots in the area reported an ash cloud that had risen up to over 10 km above sea level. Because there is so much air traffic going through the region, reports like that are an important part of monitoring volcanic activity in remote areas. Whereas the eruption had stopped within an hour or two, activity at the Alaska Volcano Observatory certainly wouldn't have.

Data had to be analysed, statements had to be published and scientists were looking for signs of any unrest that may have preceded the eruption. Indeed, looking back through the data, the volcanologists realised that Bogoslof had been showing signs of activity throughout the month of December, and the first explosion may have occurred as early as 16th December. So what kind of data can volcanologists use to monitor Bogoslof?

Even though there are no seismometers on the island itself, nearby Okmok and Makushin volcanoes have extensive monitoring networks. Because seismometers are very sensitive instruments, and volcanic eruptions make the ground shake with waves that can travel a long way, it is actually possible to look at seismic signals from Bogoslof on other islands.

Similarly, microphones recording "infrasound" (i.e. sound at frequencies much lower than the range we can detect with our ears) can detect pressure signals coming from far away, and volcanic eruptions often produce distinct infrasound.

Satellite image show the ash cloud at Bogoslof Volcano on 18th January 2017. Image: NASA Earth Observatory/Jeff Schmaltz

Satellites are also quite useful. A volcanic ash cloud can often be detected from space. Some satellites capture light of many different wavelengths, others can detect different types of gases in the atmosphere, some of which can be traced back to volcanoes. Visual observations by pilots, local residents or fishermen help to complement the picture we get from satellites.

Last but not least, volcanic lightning (i.e., lightning strikes in or around the ash cloud coming up in an eruption) has been an increasingly valuable tool to detect volcanic eruptions over the last few years. Volcanic lightning is still not fully understood and subject to active study by volcanologists around the world, but even without a complete understanding of the exact mechanism it is a spectacular sight and can be used for eruption detection. You can watch lightning happen all around the world through the World Wide Lightning Location Network if you're interested, almost in real time.

Spectacular eruption with volcanic lightning at Mt. Etna, Italy. Photo: Karl-Ludwig Poggemann

At the time of writing this post, Bogoslof continues to have explosions every few hours to days, and scientists are analysing these eruptions through all the different types of data mentioned above, even though there are no instrument directly on the volcano. Pretty amazing, isn't it?