Multi-scale Seismic Anisotropy of the Rotondo Granite: Linking Deformation Fabrics to Wave Propagation

We investigate the seismic anisotropy of the Rotondo granite (Gotthard Massif, Swiss Alps) by integrating geological and geophysical data from lab to field scale. We compare our modeled anisotropic properties with decameter measurements from boreholes and kilometer-scale regional seismicity data from the Bedretto Underground Laboratory for Geosciences and Geoenergies, demonstrating clear links between deformation fabrics and observed seismic anisotropy across scales. 

Using field- and micro-scale analyses of deformation styles and fabric orientations, we delineate discrete structural domains characterized by varying strain intensities and fabric types, ranging from isotropic granite to fractured zones or proto-mylonitic shear zones. Proto-mylonitic zones exhibit strong phyllosilicate SPO and higher percentage of Vp anisotropy (~8-27%, range is dependent on compositional variations). Fractured zones vary in frequency within the Rotondo massif and also exhibit elevated Vp anisotropy (>7.5%). For each structural domain, we compute effective elastic stiffness tensors (or 'rock recipes') to characterize their intrinsic seismic velocities. We introduce a new approach for combining multiple lithological “rock recipes” that emphasizes collective impact on bulk anisotropy and spatial context, rather than volume-weighted averaging. 

We observe a scale-dependent shift in anisotropic influence, where the control of ductile fabrics (<20 m) is progressively superseded by fractures as the observational scale increases. When these heterogeneous fabrics are aggregated, destructive interference among strongly anisotropic components reduces the bulk anisotropy to ~2.5%, which is below laboratory measured values. We find good agreement between our theoretical results and cross-borehole effective Vp measurements within the Bedretto Lab. We also find qualitative evidence for anisotropy between event relocation models (e.g. Double Difference or NonLinLoc) of background seismicity in the region at the 1-5 kilometer scale. These results demonstrate consistency in seismic anisotropy estimation across methods and scales, and show the utility of geologically-based anisotropy characterization.