REGAL: a permanent GPS network in the western Alps. Configuration and first results

The kinematics of the present-day deformation in the western Alps is still poorly known, mostly because of a lack of direct measurements of block moti on and internal deformation. Geodetic measurements have the potential to pr ovide quantitative estimates of crustal strain and block motion in the Alps , but the low expected rates, close to the accuracy of the geodetic techniq ues, make such measurements challenging. Indeed, an analysis of 2.5 years o f continuous GPS data at Torino (Italy), Grasse (France), and Zimmerwald (S witzerland), showed that the present-day differential motion across the wes tern Alps does not exceed 3 mm/yr [Calais, 1999]. Continous measurements pe rformed at permanent GPS stations provide unique data sets for rigorously a ssessing crustal deformation in regions of low strain rates by reducing the amount of time necessary to detect a significant strain signal, minimizing systematic errors, providing continuous position time series, and possibly capturing co- and post-seismic motion. In 1997, we started the implementat ion of a network of permanent GPS stations in the western Alps and their su rroundings (REGAL network). The REGAL network mostly operates dual frequenc y Ashtech Z12 CGRS GPS stations with choke-ring antennae. In most cases, th e GPS antenna is installed on top of a 1.5 to 2.5 m high concrete pilar dir ectly anchored into the bedrock. The data are currently downloaded once dai ly and sent to a data center located at Geosciences Azur, Sophia Antipolis where they are converted into RINEX format, quality checked, archived, and made available to users. Data are freely available in raw and RINEX format at http://kreiz.unice.fr/regal/. The GPS data from the REGAL network are ro utinely processed with the GAMIT software, together with 10 global IGS stat ions (KOSG, WZTR, NOTO, MATE, GRAZ, EBRE, VILL, CAGL, MEDI, UPAD) that serv e as ties with the ITRF97. We also include the stations ZIMM, TORI, GRAS, T OUL, GENO, HFLK, OBER because of their tectonic interest. We obtain long te rm repeatabilities on the order of 2-3 mm for the horizontal components, 8- 10 mm for the vertical component. Using a noise model that combines white a nd coloured noise (flicker noise, spectral index 1), we find uncertainties on the velocities ranging from 1 mm/yr for the oldest stations (ZIMM, GRAS, TOUL, TORI, SJDV) to 4-5 mm/yr for the most recently installed (CHAT, MTPL ). Station velocities obtained in ITRF97 are rotated into a Eurasian refere nce by substracting the rigid rotation computed from ITRF97 velocities at 1 1 central European sites located away from major active tectonic structures (GOPE, JOZE, BORl, LAMA, ZWEN, POTS, WETT, GRAZ, PENC, Effelsberg, ONSA). The resulting velocity field shows residual motions with respect to Eurasia lower than i mm/yr We obtain at TORI, in the Po plain, a residual velocity of 2.3 +/-0.8 mm/yr to the SSW and a velocity of 1.9 +/-1.1 mm/yr at SJDV, on the Alpine foreland. These results in --dicate that the current kinemat ic boundary conditions across the western Alps are extensional, as also sho wn by the SJDV-TORI baseline time series. We obtain at MODA (internal zones ) a residual velocity of 1.2 +/-1.2 mm/yr to the SSE. The MODA-FCLZ baselin e show lengthening at a rate of 1.6 +/-0.8 mm/yr. These results are still m arginally significant but suggest that the current deformation regime along the Lyon-Torino transect is extension, as also indicated by from recent se ismotectonic data. It is in qualitative agreement with local geodotic measu rements in the internal zones (Briancon area) but excludes more than 2.4 mm /yr of extension (FCLZ-MODA baseline, upper uncertainty limit at 95% confid ence). Our results indicate a different tectonic regime in the southern part of th e western Alps and Provence, with NW-SE to N-S compression. The GRAS-TORI b aseline, for instance, shows shortening at a rate of 1.4 +/-1.0 mm/an. This result is consistent with seismotectonic data and local geodetic measureme nts in these areas. The Middle Durance fault zone, one of the main active f aults in this area, is crossed by the GINA-MICH baseline, which shows short ening at a rate of 1.0 +/-0.8 mm/yr. This result is only marginally signifi cant, but confirms the upper bound of 2 mm/yr obtained from triangulation-G PS comparisons. The REGAL permanent GPS network has been operating since th e end of 1997 for the oldest stations and will continue to be densified. Al though they are still close to or within their associated uncertainties, pr eliminary results provide, for the first time, a direct estimate of crustal deformation across and within the western Alps.