Ambient noise tomography of the Central Cantabrian Mountains (NW Spain). New insights from the GEOCANTABRICA-COSTA seismic network.

The Cantabrian Mountains (NW Spain) are an Alpine chain that was formed as a result of the collision between Iberia and Europe in the Cenozoic. In their central sector, the uplift of the orogen led to the exhumation of a block of Variscan -Paleozoic- basement, the reactivation of Variscan structures and the formation of new E-W oriented fractures. Moreover, the formation of the Cantabrian Mountains involved the development of a crustal root with a thickness of 45-55 km that decreases up to 30-35 km towards the west. The thickening occurs preferentially in the crust that had previously been extended during the two main rifting episodes that affected this area in the Mesozoic. At the surface, the limit between the normal and the thickened crust roughly coincides with the trace of the Ventaniella fault, a subvertical crustal structure that runs for more than 400 km both inland and offshore.

In order to obtain new insights from this complex region, it was installed a network (GEOCANTÁBRICA-COSTA, doi:10.7914/SN/YR_2019) of 13 broadband stations covering an area of 160x80 km (~40 km spacing) for 8 months. The phase cross-correlation (PCC) processing technique was used to cross-correlate daily records of the 78 station pairs. After stacking the cross-correlograms, the empirical Green’s functions and the dispersion curves were obtained. Finally, a Rayleigh wave group velocity tomography was performed, retrieving the seismic signature of the Variscan crust and allowing us to extend to the north our previous seismic ambient noise tomography and complete the tomographic model of the central Cantabrian Mountains. To reveal the structure beneath the seismic stations, we also performed ambient noise auto-correlations, successfully retrieving body-wave reflections from the crust-mantle boundary that provide new information about the limits of the crustal root.

The study area presents a lingering, low-magnitude intraplate seismic activity that increases from east to west and extends into the continental shelf. The Ventaniella fault also acts as a seismic barrier to the propagation of earthquakes towards the east while provides nucleation sites along its trace. Thus, another objective of this study was to detect and relocate the local seismicity of the Cantabrian Mountains and the Cantabrian margin activity in particular. Our preliminary catalogue of events, obtained from the automatic analysis of the real-time seismic data with SeiscompP3, comprises 54 local earthquakes. Seven of them have their epicentres in the Cantabrian margin and, as expected, all were located to the west of the Ventaniella fault.