3-D crustal VS model of western France and the surrounding regions using Monte Carlo inversion of seismic noise cross-correlation dispersion diagrams.

Due to a too sparse permanent seismic coverage during the last decades, the crustal structure of western France and the surrounding regions is poorly known. In this study, we present a 3-D seismic tomographic model of this area obtained from the analysis of 2-yr continuous data recorded from 55 broad-band seismometers. An unconventional approach is used to convert Rayleigh wave dispersion diagrams obtained from ambient noise cross-correlations into posterior distributions of 1-D V_S models integrated along each station pair. It allows to avoid the group velocity map construction step (which means dispersion curve extraction) while providing meaningful V_S posterior uncertainties. V_S models are described by a self-adapting and parsimonious parametrization using cubic Bézier splines. 1268 separately inverted 1-D V_S profiles are combined together using a regionalization scheme, to build the 3-D V_S model with a lateral resolution of 75 km over western France. The shallower part of the model (horizontal cross-section at 4 km depth) correlates well with the known main geological features. The crystalline Variscan basement is clearly associated with positive V_S perturbations while negative heterogeneities match the Mesocenozoic sedimentary basins. At greater depths, the Bay of Biscay exhibits positive V_S perturbations,which eastern and southern boundaries can be interpreted as the ocean−continent transition. The overall crustal structure below the Armorican Massif appears to be heterogenous at the subregional scale, and tends to support that both the South-Armorican Shear Zone and the Paris Basin Magnetic Anomaly are major crustal discontinuities that separate distinct domains. [ABSTRACT FROM AUTHOR]