Local earthquake and ambient noise tomography of the northern Iberian Peninsula and Pyrenees

We present new three-dimensional P- and S-wave velocity models of the northern Iberian Peninsula and Pyrenees using arrival times of local earthquakes and seismic ambient noise.

The arrival time dataset has been built in two steps. First, we have merged the existing seismic bulletins of permanent seismic networks in the region (IGN, ICGC, OMP). Second, we have compiled continuous waveforms of all temporary experiments in the region from 2010 to present and have been automatically picked using the deep-learning picker PhaseNet, substantially increasing data coverage in regions with sparse permanent instrumentation. Using this augmented arrival time dataset, we have inverted it simultaneously for P and S wave velocity structure and earthquake relocation. Since the region is too large for the flat-earth approximation used in the tomography code we have obtained multiple overlapping smaller models and calculated the final model by averaging the individual models.

To further enhance structural resolution, particularly in areas with limited earthquake ray coverage, we incorporated results from ambient noise tomography based on inter-station surface-wave dispersion measurements. These data provide complementary constraints on the shallow crust and improve lateral continuity of the velocity model, especially in aseismic regions and across major sedimentary basins.

The obtained P and S wave velocity models provide detailed images of the subsurface structure of the region, including parts that were poorly imaged before. The addition of arrival times picked at temporary stations has been of fundamental importance to illuminate the central part of the region, because of its low seismic activity and lack of permanent stations. Particularly well imaged are the sedimentary basins, including the southern Aquitaine basin, and the northern Ebro and Duero basins and their connection along the Rioja Trough.