Mapping the basement of the Cerdanya Basin (eastern Pyrenees) using seismic ambient noise

Abstract

Ambient seismic noise acquired in the Cerdanya Basin (eastern Pyrenees) is used to assess the capability of different methodologies to map the geometry of a small-scale sedimentary basin. We present results based on a 1-year-long broad-band deployment covering a large part of the eastern Pyrenees and a 2-month-long high-density deployment covering the basin with interstation distances around 1.5g€¯km. The explored techniques include autocorrelations, ambient noise Rayleigh wave tomography, horizontal-to-vertical-spectra ratio (HVSR) and band-pass-filtered ambient noise amplitude mapping. The basement depth estimations retrieved from each of these approaches, based on independent datasets and different implicit assumptions, are consistent, showing that the deeper part of the basin is located in its central part, reaching depths of 600-700g€¯m close to the Tet fault trace bounding the Cerdanya Basin to the NE. The overall consistency between the results from all the methodologies provides solid constraints to our basement depth estimation, although significant differences arise in some areas. The results also show that, when high-density seismic data are available, HVSR and ambient noise amplitude analysis in a selected frequency band are useful tools to quickly map the sedimentary 3D geometry. Beside this methodological aspect, our results help to improve the geological characterization of the Cerdanya Basin and will provide further constraints to refine the seismic risk maps of an area of relevant touristic and economic activity.