Azimuthal seismic anisotropy in the crust beneath the Granada Basin (Spain)

Abstract

In this research, we conducted the first P-wave tomographic imaging of 3-D azimuthal anisotropy of the Granada Basin (Betic Cordillera, Spain) introducing recent advances in the application of this method, thanks to abundant, high-quality data sets recorded by a dense seismic network deployed in the study area during the Atarfe-Santa Fe seismic series (2020-2021). We also determined high-resolution P- and S- wave tomography for 3-D isotropy of the study area down to 14 km depth and then the three-dimensional distributions of Poisson’s ratio (σ). The result for the uppermost crust reveals the lower velocities associated with the predominantly Neogene- Quaternary sediments in the Basin. During the Atarfe-Santa Fe seismic series there were five earthquakes with a magnitude of over 4.1. In the source areas of the first of these earthquakes, significant variations were detected in P-velocities and high Poisson’s ratios. This suggests that fluids might be involved in the nucleation and development of the seismic sequence. The fast polarization directions (NE-SW) in the central study area are mostly parallel to the Cadiz-Alicante fault system and almost perpendicular to the NW-SE fault plane directions obtained from the focal mechanism solutions for the earthquakes with the largest magnitudes, which present a clearly NE-SW extensional model. At the end of the middle Miocene, the Cadiz-Alicante fault system was practically immobilized, and an approximate NNW-SSE compression with a perpendicular extension, which was strongly pronounced in the Granada basin, was established. This extension is now active. Our results could therefore be suggesting that extension tectonics is the dominant effect in the upper crustal depth, as reflected by a significant NE-SW Fast Velocity Direction (FVD). Another possibility is that the FVD may have continued unchanged since the end of the middle Miocene due to the control exerted by the Cadiz-Alicante fault system, indicating structure-induced crustal anisotropy.