Crustal strain in the Southern Alps, France, 1948–1998

Active tectonics in the Western Alps is revealed by a moderate level of seismic activity and geological evidences for Quaternary deformation. We present new geodetic determinations of the current strain rates in the southern part of the Western Alps, that complement existing results in Provence, the Jura, and the Subalpine and Belledone massifs around Grenoble. We combined first- and second-order triangulation data collected in 1948 with global positioning system (GPS) data collected in 1998. We estimate shear strain rates of 0.1–0.2 μrad yr −1 over distances on the order of 30 km, significantly different from zero in most of the network. We obtain NW–SE to N–S compressive shear strain directions over most of the area, in agreement with the seismological and geological data. These geodetic estimates correspond to long-term overall N–S to NW–SE shortening of 2–4 mm yr −1 over the study area, on the same order as the far-field shortening measured by continuous GPS on the Grasse–Torino baseline (−2.0±0.5 mm yr −1), that incorporates our geodetic network. These results provide new constraints on interseismic strain in the Western Alps.