Preliminary results from a temporary high-resolution broadband network around Picos de Europa (Cantabrian Mountains): ambient noise tomography and seismicity distribution

The North Iberian Margin (NIM) constitutes an example of a former plate boundary where to explore the role played by geological inheritance during the alpine convergence between Iberia and Europe in the Paleogene. The convergence, which resulted in the partial and asymmetric closure of the Bay of Biscay, resolved in major tectonic differences along this boundary, depending on the previous tectonic history of the crust: short-lived south directed subduction of oceanic crust in the West under the crystalline basement of Iberia (Variscan), continental collision in the Pyrenees in the East, shortening of a previously hyperextended margin in the middle part.

The Picos de Europa massif (Cantabrian Mountains, CM), is located in this middle region between the continental collision and the arrested subduction,  and from the structural point of view represents part of the leading edge of the Variscan orogenic wedge, the forefront of the Variscan thrusts over the foreland. The area reflects a history of deposition where synorogenic thick carbonate platforms are affected by thrusting during the Variscan collision between Gondwana and Laurussia.

The dominance of carbonate rocks in Picos de Europa over the shales in the surrounding Variscan foreland sediments, together with its subsequent alpine tectonic history, contributed to its current orography. It is the area of the Cantabrian Mountains with the highest concentration of peaks above 2000 m.

A temporal local network of 10 broadband seismic stations was deployed in the area to study its seismicity and produce a high-resolution tomography of the upper crust in order to gain insight into its tectonic structure. A previous tomography at regional level, revealed the existence of a low velocity zone dipping north interpreted to represent the frontal thrust of the CM. Although scarce, seismicity associated to this major tectonic structure has an impact at the surface as Picos de Europa, in its hanging wall, is well known by the steepness of its slopes, with the main river incising over 2000 m. As well as being the first Spanish Natural Park is one of the most visited. The increasing touristic pressure over this protected space has highlighted the importance of constraining natural hazards in mountain areas.

Results from ambient noise data recorded during six months are presented. The cross-correlation technique was used to retrieve the empirical Green functions of the subsurface between all station pairs, showing the emergence of Rayleigh waves. After measuring dispersion velocities, Rayleigh-wave group velocity tomography maps were computed at different periods and then inverted in order to calculate S-wave velocities as a function of depth, reaching the upper 8-9 km of the crust. The tomography shows the presence of a relative high velocity body at 5-6 kms in the northern part of Picos de Europa, and the presence of two low velocity bodies at 6-9 km aligned NW-SE. Preliminary interpretation points to likely Variscan structures underlying the geometry of the velocity patterns.