3D deformation in the South-Western European Alps (Briançon region) revealed by 20 years of geodetic data
Abstract
The overall Briançon region undergoes a moderate but steady seismic activity with instrumental magnitudes up to Ml 5.0. Previous studies pointed out that this area is characterized by a great majority of extensional and dextral focal mechanisms, along north-south to N160 oriented faults. Current works aim at measuring and quantify the upper-crustal deformation in this seismically active area, and to decipher the seismic part of the deformation. A dense, local network of 30 GPS stations, covering a 50 × 60 km2 area has been temporarily surveyed in 1996, 2006, 2011 and 2016 by GPS, which represent one of the longest repeated set of geodetic campaigns in the European Alps. The combined analysis of the GPS temporary surveys over 20 years with the continuous GPS measurements over 18 years from the French RENAG network confirms horizontal velocity amplitudes below 1 mm/year within the local network. The long observation interval and the redundancy of the dense campaign network measurement help to constrain a significant local deformation pattern in the Briançon region, yielding an average E-W extension of 22 ± 8 nanostrain/year, coherent with but more precise than previous results obtained before the 2016 measurements. The direction and amplitude of the geodetic rate is both qualitatively and quantitatively coherent with the deformation rate previously derived from 37 years of seismic data (1970-2007). The next step of this project will be to reassess the seismic stress/strain with an up to date database all over the western Alps to better constrain the geodetic vs. seismic deformation rates. A good vertical accuracy is also revealed for the first time in the area by the long time span of temporary GPS observations. We find that the accuracy of vertical measures depends on the type of geodetic marker used, namely screw marks and classical bolt marks, the first ones being well coherent with permanent station velocities. Our new integrated analysis (including the 2016 survey) confirms a vertical average rate of 0.78 ± 0.5 mm/yr over 10 years of measurement for the 19 screw marks. The uplift of the core (so-called high chain) of the Alpine belt in this extensional regime has already been observed on a larger scale by permanent GPS stations, and is well discussed. Thanks to the forced antenna centering, dense campaign networks are able to provide significant constraints on geodynamic models of the Alps, with a complementary spatial coverage with respect to permanent GPS networks.
- Publication:
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EGU General Assembly Conference Abstracts
- Pub Date:
- April 2018
- Bibcode:
- 2018EGUGA..2010415M