Abstract
Seismicity can be used to better understand interactions between magma bodies, hydrothermal systems and their host rocks - key factors influencing volcanic unrest. Here, we use earthquake data to image, for the first time, the seismic velocity structure beneath Aluto, a deforming volcano in the Main Ethiopian Rift. Traveltime tomography is used to jointly relocate seismicity and image 3D P- and S-wave velocity structures and the ratio between them (VP/VS). At depths of 4-9 km, the seismicity maps the top of a large low velocity zone with high VP/VS, which we interpret as a more ductile and melt-bearing region. A shallow (< 3km) hydrothermal system exhibits low seismic velocities and very low VP/VS (~1.40), consistent with the presence of gases exsolved from a deeper melt-rich mush body. The Artu Jawa fault and fracture system provides the migration pathway that connects the deeper mush body with the shallow hydrothermal system. Together, these observations demonstrate that the interaction between magmatic and hydrothermal systems, driven by the exchange of fluids, is responsible for the restless behaviour of Aluto.
Highlights
Interactions between magmatic and hydrothermal systems beneath volcanoes are poorly understood, but play a crucial role in eruptions, especially those that are phreatomagmatic (Pritchard et al, 2019; Troise et al, 2019)
In agreement with Wilks et al (2017), most events are located above sea level, beneath the volcano edifice
We interpret the region of low VP/VS ratios that lies above sea level in terms of an over-pressurized gas-rich volume (e.g., Dvorkin et al, 1999; Lees and Wu, 2000) occupying highly fractured and hydrothermally altered volcanic products
Summary
Interactions between magmatic and hydrothermal systems beneath volcanoes are poorly understood, but play a crucial role in eruptions, especially those that are phreatomagmatic (Pritchard et al, 2019; Troise et al, 2019). Seismic velocities and their ratio (VP/VS), can be used to image fault structures and regions of partial melt and over-pressured gases (e.g., Johnson and Polland, 2013; Muksin et al, 2013). Such observations have implications for understanding volcanic unrest, assessing volcanic hazard and optimizing geothermal exploration
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