Seismic anisotropy tomography beneath La Palma in the Canary Islands, Spain

Abstract

We investigated the detailed 3-D crustal structure beneath the island of La Palma (Spain) using the data from local earthquakes. We separately analyzed the pre-eruptive and the sin-post-eruptive seismic activity as well as the complete dataset. For each set we performed a separate isotropic seismic tomographic inversion to obtain isotropic 3-D P-wave and S-wave velocity (Vp, Vs) models and Vp/Vs ratio distribution down to 15 km depth. We also performed two types of anisotropic tomography using the same datasets to study 3-D Vp azimuthal and radial anisotropy structures. A high-Vp anomaly was revealed under the Taburiente caldera, which coincides with a resistive body interpreted as the oldest basal complex. In the shallower layers under the southern part of the island, low-V anomalies in the north-south direction were identified. These are associated with the N-S rift structure in Cumbre Vieja in accordance with gravity and geoelectric anomalies. Both electrical and seismic anomalies around the Cumbre Vieja volcanic ridge could indicate the presence of an active geothermal system. Just beneath the active volcano, we obtained one of the most prominent features of the seismic structure, i.e., an elongated high Vp/Vs anomaly located beneath the northwestern part of the Cumbre Vieja. These results can be interpreted as magma reservoirs containing high degrees of partial melt and/or fluids. Vp azimuthal anisotropy in the study area is mostly governed by pervasive regional and structural characteristics, for example, running parallel to the APM direction of the African plate in the western part of the island and almost parallel to the tensional stress direction in the southwestern corner. Negative Vp radial anisotropy generally appears in the crust under the Cumbre Vieja volcano down to 10 km depth. This zone is probably caused by local asthenospheric upwelling at shallow depths, possibly associated with rift-related lithospheric extension.