Researchers from the national observation service Résif-Rénag have highlighted complex deformation patterns through long-term GPS measurements.
Most of the permanent GPS (Global Positioning System) stations in the Western Alps now have more than 10 years of continuous measurements. Their individual horizontal velocities converge to amplitudes of less than 0.3 mm/year with respect to the stable European plate. Intermittent GPS measurements over a period of 22 years, as well as several independent geodetic solutions that constrain the speeds of permanent GPS stations covering up to 16 years of observation, make it possible to determine a velocity field despite extremely low rates of displacement. This available velocity field is an opportunity to highlight persistent (and therefore reliable) deformation patterns in the Western Alps, which will subsequently allow a realistic interpretation of the tectonic activity in the area.
In order to assess the accuracy of intermittent measurements, the speeds of the field stations were compared with those of the nearest permanent stations. The differences were 0.16, 0.22 and 1.65 mm/year for the North, East and Vertical components respectively, highlighting the good performance of the intermittent stations on the horizontal components. The accuracy of the permanent measurements was evaluated by superimposing two independent velocity solutions, calculated using two different approaches. The first approach consists in forming double differences in GPS observations, which eliminates a number of sources of error and leads to a relative positioning between the stations (GAMIT routine). The second approach, called Precise Point Positioning (PPP), estimates the sources of error (otherwise eliminated by the double differences) and gives an absolute position of each station (CSRS-PPP routines). The differences between the two velocity fields based on these complementary methods are estimated to be 0.15 mm/year on the horizontal components and 0.44 mm/year on the vertical.
While the individual speeds of the GPS stations are still too uncertain to be interpreted, a regional deformation pattern could be identified. It is based on all intermittent and permanent measurements in the Western Alps. This pattern corresponds to an east-west extension already shown by preliminary GPS results in 2002 and also visible locally in a dense GPS network in the inner Western Alps (in the Briançonnais). However, the recently observed deformation amplitudes are now 10 times lower than those published in the 2000s: 0.6 nanostrain/year over a 150 km wide area and 2.6 nanostrain/year over a distance of 50 km in the centre of the massif (1 nanostrain/year corresponds to a velocity difference of 0.1 mm/year over a distance of 100 km). This observation illustrates the convergence of Alpine GPS speeds towards low values over long measurement times.
The number of permanent and intermittent GPS stations included in the analyses makes it possible to identify deformation patterns with increased spatial resolution, while continuing to exploit redundancy between nearby stations. Thus, the analysis of GPS velocities projected on profiles across the massif shows both an area of extension in the centre of the massif (12.5-15.3 nanostrain/year in the northern and central part; 3.1-3.3 nanostrain/year in the southern part), but also shortening along the eastern and western edges of the massif (2.6-8.1 and 1.3-1.5 nanostrain/year in the northern and central part, and in the southern part, respectively). This result is confirmed and reinforced by the comparison of the double difference solution with that calculated in PPP. This high-contrast geodetic deformation pattern is consistent with earthquake mechanisms in the Western Alps as well as with the deformation and stress patterns corresponding to this seismic activity. The advantages of GPS measurements are that they allow 1) the quantification of deformation rates whose seismicity could only be deduced from the style (extension, shortening, lateral shift), 2) their regionalisation thanks to the density of the combined network of permanent/intermittent stations, and 3) the measurement of low deformation on a continuous basis, whereas seismological measurements are dependent on the occurrence of relatively rare earthquakes in the Alps.
In order to understand the driving force behind the weak but persistent seismicity in the Western Alps, it is important to identify the process behind this deformation pattern. Indeed, the vertical movement observed in the heart of the Alpine chain (overrection between 2.0 mm/year in the north and 0.5 mm/year in the south of the Western Alps) imposes strong constraints on the possible drivers of the current deformation. The discovery of this overstraining effectively eliminates two hypotheses: that of an extensive movement between two distinct tectonic units, as it would be linked to subsidence, and that which would explain the extension observed by a gravity collapse of the Western Alps Massif having completed its formation and growth phase. The most likely hypotheses would therefore be volume forces lifting the Alps, related to 1) the discharge of the weight of glaciers by melting ice; 2) the discharge by erosion of a rock mass; 3) abnormal structures and flows in the crustal and lithospheric root, leading to dynamic support of the Alps.
Finally, the available GPS measurements show first evidence of a spatial separation between the maximum overrection and the maximum extension and seismicity. If this phenomenon is confirmed by longer measurement series, it calls for a combination of different processes to explain the current dynamics in the Western Alps.
Does Long-Term GPS in the Western Alps Finally Confirm Earthquake Mechanisms ?, A. Walpersdorf, L. Pinget, P. Vernant, C. Sue, A. Deprez, and the RENAG team, Tectonics, Sept. 2018, doi.org/10.1029/2018TC005054
Scientific contact at ISTerre
This news was also relayed by