Shear-wave attenuation anisotropy: a fluid detection tool

The behaviour of fluids in preferentially aligned fractures plays an important role in a range of dynamic processes within the Earth. In the near-surface, understanding systems of fluid-filled fractures is crucial for applications such as geothermal energy production, monitoring CO2 storage sites and exploration for metalliferous sub-volcanic brines. Mantle melting is a key geodynamic process, exerting control over its composition and dynamic processes. Upper mantle melting weakens the lithosphere, facilitating rifting and other surface expressions of tectonic processes. Aligned fluid-filled fractures are an efficient mechanism for seismic velocity anisotropy, requiring very low volume fractions, but such rock physics models also predict significant shear-wave attenuation anisotropy. Here we demonstrate a new method for measuring shear-wave attenuation anisotropy and apply it to synthetic examples and to teleseismic SKS phases recorded at the station FURI, in Ethiopia. At FURI we measure attenuation anisotropy which can only be explained by the presence of aligned fluids, most probably melts, in the upper mantle. Modelling of this result suggests that melt aligned in fractures dipping ca. 40° that strike perpendicular to the Main Ethiopian Rift, are required to explain the observed attenuation anisotropy. These results show that attenuation anisotropy could be a useful tool for discriminating between anisotropy due to crystal or melt alignment, and may offer strong constraints on the extent and orientation of melt inclusions.