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New insights into how continuous seismic signals announce an eruption

In the recently published Nature Geoscience paper a diverse group of scientists based at DIAS and other research institutes in Ireland and Iceland developed a new understanding of seismic signals prior to an eruption. The focus of the study is an eruption in Iceland in 2014/15 that was preceded by two weeks of increased, migrating seismicity. This seismicity is the noise of the breaking crust at depth and gave scientists the possibility to ‘watch’ how magma propagated horizontally until it eventually made it to the Earth’s surface. However, the puzzling observation was that no earthquakes occurred at less than 3 km depth, although magma passed through this region. We found that a long-lasting continuous seismic signal, called tremor, exists at this depth instead. This tremor was usually understood as being caused by moving fluids, but it seems that it consists instead of millions of tiny earthquakes that are so closely spaced that they merge into one another and appear as tremor. It seems that the uppermost part of the crust is too weak to generate big earthquakes and it therefore breaks through many small earthquakes. In our paper we describe how the crust beneath the ice opened little by little in about 19 hours at a speed of 220 m/h. As such eruptions beneath ice can distribute huge amounts of ash in the air, understanding these signals is important for volcano monitoring and eruption early-warning.