Recently completed PhD student Eva Eibl has three papers published

In the recently published Nature Geoscience paper a diverse group of scientists, including recently completed PhD student Eva Eibl (UCD School of Earth Sciences), 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 it gave scientists the possibility to ‘watch’ how magma propagated horizontally until it eventually made it to the Earth’s surface. They observed that no earthquakes occurred at less than 3 km depth, although magma passed through this region. Instead they found that a long-lasting continuous seismic signal, called tremor, exists at this depth. This tremor was usually understood as being caused by moving fluids, but it now seems that it consists 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 their paper they describe how the crust beneath the ice opened little by little over the space of 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.

In the recently published Geophysical Journal International paper titled 'Helicopter location and tracking using seismometer recordings' they used frequency domain methods usually applied to volcanic tremor to analyse ground based seismic recordings of a helicopter. The abstract states: We preclude misinterpretations of tremor sources and show alternative applications of our seismological methods. On a volcano, the seismic source can consist of repeating, closely spaced, small earthquakes. Interestingly, similar signals are generated by helicopters due to repeating pressure pulses from the rotor blades. In both cases the seismic signals are continuous and referred to as tremor. As frequency gliding is in this case merely caused by the Doppler effect, not a change in the source, we can use its shape to deduce properties of the helicopter and its flight path. We show in this analysis that the number of rotor blades, rotor revolutions per minute, helicopter speed, flight direction, altitude and location can be deduced from seismometer recordings. Access to GPS determined flight path data from the helicopter offers us a robust way to test our location method.

In the Journal of Geophysical Research paper they analyzed a type of earthquake that is long lasting, has no sudden start but is slowly getting stronger, and is recorded during eruptions. This so-called tremor is usually thought to be merely caused below the vent where the magma moves up toward the surface. Therefore, it was, for example, used to estimate how much magma and how fast the magma erupts, how high, and where the erupted magma would go. However, using a dense network of seismometers at only 15 km distance from the erupting magma—during the Holuhraun eruption in Iceland in 2014/2015— they found three areas where tremor is caused. These are (i) below the vent where the magma moves up toward the surface, (ii) at the cooling sides of the growing lava flow field, and (iii) probably at less than 2 km depth where the Earth's crust breaks and magma moves horizontally. They warn other scientists to be careful when using the tremor amplitude but also show that we can with our instruments watch in real time where the lava flow field is growing.