Active deformation in the inner western Alps inferred from comparison between 1972-classical and 1996-GPS geodetic surveys

Abstract Eighteen geodetic points surveyed in 1972 by the French National Geographic Institute (IGN) were remeasured by GPS in 1996 in the Brianconnais and Piemont Zones, east of the Pelvoux massif (French Western Alps). A displacement vector set was determined for the two surveys' common points. Calculations of the strain-rate tensors associated with 15 triangular cells of the network have been performed. Only four of them show a strain rate significant at a 95% level of confidence. These data suggest an E–W extension of about 2–4 mm/yr between the western and eastern part of the network (Pelvoux external crystalline massif and Queyras blueschists, respectively) associated with N–S shortening. This active deformation agrees with neotectonic and seismotectonic data. The measured tectonic motion seems to be distributed throughout the central part of the Brianconnais zone, where the seismic activity is concentrated. The local seismicity has been precisely surveyed since 1989. It is moderate (Ml

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