Seismic tomography of Aluto volcano: insights into subsurface fluid distribution

SUMMARY Understanding subsurface fluid distribution in volcanic reservoirs is critical for geothermal energy development, critical mineral exploration and forecasting eruptions. Here, we use traveltime tomography to image the seismic velocity structure beneath Aluto volcano, the first pilot geothermal project in Ethiopia, located in the Main Ethiopian Rift. Using seismic data recorded from January 2012 to January 2014, we invert for the 3-D P-wave (Vp), S-wave (Vs) and Vp/Vs ratio. To reduce the non-uniqueness in interpretation, we also compare our results with previously published work on attenuation tomography and magnetotelluric images. Elevated Vp/Vs ratios [at 0 km below sea level (bsl)] around productive geothermal wells suggest high fluid content and/or elevated temperature. Vp/Vs values above 1.8 are observed along the caldera rims and hydrothermal vents, indicating fault and fracture systems as primary fluid conduits. High Vp/Vs below 6 km bsl likely reflects high-temperature areas or the presence of partial melt. In contrast, low Vp/Vs ($\lt 1.5$), low Vp, and average to high Vs beneath the caldera at around 5 km bsl is interpreted as a crystallized body with overpressurized gas volume formed during phase separation and transported upward through fractures and fault systems, accumulating at shallower levels. These findings highlight fluid pathways through the caldera rims and faults, with volatile-rich partial melt at greater depth beneath the caldera centre. Traveltime tomography thus offers a valuable constraints on subsurface fluid distribution and is valuable tool in geothermal exploration.