Human activity causes vibrations that propagate into the ground as high-frequency seismic waves. Measures to mitigate the COVID-19 pandemic caused widespread changes in human activity, leading to a months-long reduction in seismic noise of up to 50%. The 2020 seismic noise quiet period is the longest and most prominent global anthropogenic seismic noise reduction on record. While the reduction is strongest at surface seismometers in populated areas, this seismic quiescence extends for many kilometers radially and hundreds of meters in depth. This provides an opportunity to detect subtle signals from subsurface seismic sources that would have been concealed in noisier times and to benchmark sources of anthropogenic noise. A strong correlation between seismic noise and independent measurements of human mobility suggests that seismology provides an absolute, real-time estimate of population dynamics.

The Alpine collision between the Iberian and European plates resulted in a complex crustal structure beneath the northern Iberian Peninsula, as revealed from a new set of seismic refraction/wide‐angle reflection profiles. The study area is characterized by two major E‐W ranges, the Cantabrian Mountains and the Pyrenees, which are relayed to the south by the Iberian Chain. Important variations in crustal thickness and velocity distribution are found in a 560‐km‐long E‐W transect. In contrast to the typical 30‐ to 32‐km‐thick European Variscan crust of the western end of the profile, a continuous Alpine crustal root is evidenced from the Cantabrian Mountains to the central Pyrenees, with a Moho depth of 46–48 km, locally rising to ∼40 km depth in between, beneath the Basque‐Cantabrian Basin. Another outstanding feature is the inferred presence of high velocities of 6.40–6.75 km/s at midcrustal depths, which can be associated with portions of a lower crustal wedge from the northern (European) domain indenting the southern (Iberian) crust during the Alpine stage of compression. This indentation produces the delamination of the Iberian crust, with northward underthrusting of its lower half and the consequent crustal thickening. The indenting wedge has a discontinuous presence along the longitudinal section, as it was controlled and/or affected by N‐S to NE‐SW structures. Further evidence of the northward subduction of the Iberian plate is provided by another profile toward the Aquitaine Basin, while a N‐S profile across the Iberian Chain reveals an Alpine midcrustal thickening beneath this belt with Moho depths of ∼42 km.

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We exploit the data from five seismic transects deployed across the Pyrenees to characterize the deep architecture of this collisional orogen. We map the main seismic interfaces beneath each transect by depth migration of P-to-S converted phases. The migrated sections, combined with the results of recent tomographic studies and with maps of Bouguer and isostatic anomalies, provide a coherent crustal-scale picture of the belt. In the Western Pyrenees, beneath the North Pyrenean Zone, a continuous band of high density/velocity material is found at a very shallow level (~10 km) beneath the Mauleon basin and near Saint-Gaudens. In the Western Pyrenees, we also find evidence for northward continental subduction of Iberian crust, down to 50–70 km depth. In the Eastern Pyrenees, these main structural features are not observed. The boundary between these two domains is near longitude 1.3 °E, where geological field studies document a major change in the structure of the Cretaceous rift system, and possibly a shift of its polarity, suggesting that the deep orogenic architecture of the Pyrenees is largely controlled by structural inheritance.