A Seismic Investigation of the Magmatic and Hydrothermal Systems of Nabro Volcano and other East African Rift Volcanoes

One of the fundamental objectives of volcano seismology is to develop an understanding of the dynamics of active magmatic systems. By studying the seismicity of a volcano, it is possible to map the volcanic subsurface:
determining what drives and feeds the magmatic system and tracing its evolution through time. This has important implications for assessing volcanic hazard and predicting eruptive behaviour. Understanding how volcanic fluids
interact is also vital for evaluating a volcano's geothermal energy production potential. Seismic tomography is a powerful geophysical tool used to image the Earth's interior at a variety of scales. The
technique inverts seismic data to constrain the 2D or 3D P- and S-wave velocity structures and the ratio between them (Vp/Vs). It has been frequently applied on a local scale to deforming volcanoes, in order to better understand
the subsurface active magmatic processes responsible for volcanic unrest (e.g. Greenfield et al., 2016; Wilks et al., 2020). Knowledge of the seismic velocity structure beneath a volcano helps to identify melt-bearing regions and
fluids at depth, constraining interactions between magmatic and hydrothermal systems. Shear wave splitting is another seismic method that is being increasingly applied to the investigation of volcanological
processes, particularly stress variations in the subsurface (e.g. Nowacki et al., 2018). Shear wave splitting occurs when seismic waves propagate through an anisotropic material; by measuring the resulting variation in
seismic wave speed and direction of propagation, the anisotropy of the subsurface can be characterised. Measuring seismic anisotropy provides an indication of dynamic processes, such as deformation, which are invisible to other
techniques (Wookey, 2012). Nabro is an off-rift volcano located on the central part of the Danakil microplate near the Ethiopia-Eritrea border (Hamlyn et al., 2014). Despite the fact that Nabro erupted in 2011, the structure of its magma plumbing system remains a mystery. In general, it is still unclear how magma is supplied to such volcanoes offset from the axis of spreading; further, their role in accommodating extension is not well understood (Maccaferri et al., 2014).
A seismic network established around Nabro from August 2011 - October 2012 yielded 14 months of seismic data. These data have been partially analysed previously by Hamlyn et al. (2014); however, most of the seismicity
has not been catalogued, due to the time-consuming nature of manually picking P- and S-wave arrivals. A new deep learning model for automating phase arrival detection, based on convolutional neural networks, has recently
been applied to the seismic data from Nabro (Lapins et al., 2020). This has produced a catalogue of 33,950 events (Figure 1), far more than the previous catalogues of manually-identified events, indicating that Nabro is a prime
candidate for both a seismic tomography inversion and a shear wave splitting anisotropy analysis. As well as Nabro, there are other volcanoes in the East African Rift System that would be interesting targets for
a seismic investigation into their subsurface processes. For example, both Corbetti and Aluto volcanoes have been seismically monitored over several months and hence there is a wealth of unanalysed seismic data to exploit. Aluto
is currently a viable and productive geothermal resource (Hochstein et al., 2017), and there are plans to establish a geothermal power plant at Corbetti; an analysis of the hydrothermal and magmatic systems of these volcanoes will
help optimise geothermal production.