Quantification of the seismic strain in the western Alps

The western alpine tectonic regime is characterized by ongoing widespread extension in the highest zones of the belt and transpressive/compressive tectonics at the external limits of the belt [Sue et al., 1999, Delacou et al, submitted]. The overall geodetic strain corresponds also to a radial extension across the western Alps [Vigny et al., 2001] and the data of the permanent GPS networks in western Europe provided an alpine strain pattern pretty close to the strain/stress states inferred from seismotectonics [Calais et al., 2002]. Thus, there is a quite good qualitative coherency between seismotectonic and geodetic approaches. We attempt here to quantify the seismic part of the deformation. The aim is to compare the seismic strain with the overall geodetic strain, at the local and global scales. We used the regionalization of the seismic deformation and the stress inversion of Delacou et al. [submitted] to determine sub-areas of quite homogeneous seismic strain. In each sub-area, we computed the total seismic moment tensor, and the associated yearly seismic strain rate. This rate allows to compare different areas of the belt, in which the seismic catalogue may cover various range of time. Thus we can estimate the seismic strain per sub-area in the whole belt. To improve this approach, we use typical magnitude distributions (b-values) in large areas of the belt [e.g. Sue et al., 2002] to take into account the part of the deformation which corresponds to the smaller earthquakes (without focal mechanisms). A second step of improvement will be to take into account historical catalogues available in the Alps to derive the seismic strain on larger time scale, and to compare the seismic strains of the historical vs. the instrumental periods. This preliminary study brings new quantitative elements to understand the ongoing geodynamic processes in the alpine belt. The low seismic strain rates we obtain in a belt characterized by a high tectonic contrast in a quite limited area, could suggest that the Alps are currently in a meta-stable state, from a tectonic viewpoint, ruled by isostasy/buoyancy forces rather than European/Apulia plate tectonic collision. REF: Calais et al., 2002, Geology, 30-7, 651-654; Delacou et al., submitted to GJI; Sue et al., 1999, JGR, 104, 25611-25622; Sue et al., 2001, Tectonophysics, 320, 17-29; Sue et al. 2002, GRL, 29 (8), 10.1029; Vigny et al., 2001, J. Geodesy, 76, 63-76.