Characterising hydrothermal fluid pathways beneath Aluto volcano, Main Ethiopian Rift, using shear wave splitting

Andy Nowacki,Matthew Wilks,J.-Michael Kendall,Juliet Biggs,Atalay Ayele

doi:10.1016/j.jvolgeores.2018.03.023

Abstract

Geothermal resources are frequently associated with silicic calderas which show evidence of geologically-recent activity. Hence development of geothermal sites requires both an understanding of the hydrothermal system of these volcanoes, as well as the deeper magmatic processes which drive them. Here we use shear wave splitting to investigate the hydrothermal system at the silicic peralkaline volcano Aluto in the Main Ethiopian Rift, which has experienced repeated uplift and subsidence since at least 2004. We make over 370 robust observations of splitting, showing that anisotropy is confined mainly to the top ∼3 km of the volcanic edifice. We find up to 10% shear wave anisotropy (SWA) is present with a maximum centred at the geothermal reservoir. Fast shear wave orientations away from the reservoir align NNE–SSW, parallel to the present-day minimum compressive stress. Orientations on the edifice, however, are rotated NE–SW in a manner we predict from field observations of faults at the surface, providing fluid pressures are sufficient to hold two fracture sets open. These fracture sets may be due to the repeated deformation experienced at Aluto and initiated in caldera formation. We therefore attribute the observed anisotropy to aligned cracks held open by over-pressurised gas-rich fluids within and above the reservoir. This study demonstrates that shear wave splitting can be used to map the extent and style of fracturing in volcanic hydrothermal systems. It also lends support to the hypothesis that deformation at Aluto arises from variations of fluid pressures in the hydrothermal system. These constraints will be crucial for future characterisation of other volcanic and geothermal systems, in rift systems and elsewhere.

Highlights

IntroductionGeothermal resources worldwide are very frequently associated with active or recently-active volcanoes (e.g., Glassley, 2010)
We find pervasive splitting which does not increase with depth, showing that shear wave anisotropy of between 0.2% and 10% is present, and is confined to the top 5 km or less
The fast orientations we see outside the main edifice align with the Wonji Fault Belt, a series of faults which accommodate present day strain, consistent with these fractures being used as conduits of fluids in the near surface

Summary

Introduction

Geothermal resources worldwide are very frequently associated with active or recently-active volcanoes (e.g., Glassley, 2010). In these locations, a magmatic heat and fluid source is present, and an accompanying hydrothermal system has developed (e.g., Grant and Bixley, 2011), which permits the extraction of heat for immediate use or electricity generation. Locations where recently-active volcanoes can be studied in terms of their geothermal potential and volcanic history include the world’s major rift systems, such as Iceland (Arnórsson, 1995), northern New. Zealand (Wilson and Rowland, 2016), and the East African Rift, which we study here. The surface expression of this magmatism can be seen at the silicic volcanic centres within the MER, which frequently host calderas (e.g., Mohr, 1962; WoldeGabriel et al, 1990) and which are of substantial interest for current and future geothermal energy production

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