https://www.abovethenormnews.com/2025/08/07/western-alps-deformation/


The Western Alps Are Quietly Tearing Apart



For decades, the Western Alps were considered one of Europe’s most visually dramatic yet structurally stable mountain chains. Although not immune to seismic events, its moderate activity and relatively low magnitude earthquakes did little to raise public alarm. That may change. A new geodetic study based on 25 years of high-precision GNSS (Global Navigation Satellite System) measurements has revealed a pattern of tectonic movement in the Western Alps that confirms two major fault systems are not only active but deforming at quantifiable, measurable rates. These movements are not just abstract data. They represent real forces shaping the terrain beneath towns, infrastructure, and population centers in both France and Italy.

The study, led by Andrea Walpersdorf and her team at ISTerre and the University of Grenoble Alpes, is the most detailed GNSS-based analysis of this region to date. It focuses on two critical structures: the High Durance Fault (HDF) in the internal zones of the Alps, and the Belledonne Fault (BDF) in the external zones closer to the foreland. These faults, until now treated largely in separate contexts, are shown to be mechanically and kinematically linked. Their coordinated activity reveals a dynamic tectonic scenario in a region previously considered too slow-moving to pose serious seismic threats in the near term.

By integrating over two decades of both permanent and campaign-based GNSS data, the researchers were able to measure crustal deformation rates in the range of 0.1 to 1 millimeter per year. This may sound minor, but in tectonic terms, it is substantial. Slow rates like these mean that large amounts of strain can silently build up over decades or centuries, only to be released suddenly during seismic events.

The High Durance Fault, which cuts through the Briançonnais region, was found to be undergoing an extensional movement estimated at 19 to 25 nanostrain per year. This corresponds to a slip rate of up to 0.39 millimeters per year in extensional motion, along with a significant component of right-lateral (dextral) strike-slip motion. The Belledonne Fault, by contrast, exhibits a transpressive mode of deformation. It moves laterally at roughly 0.2 millimeters per year but also compresses, albeit at lower strain rates of about 5 nanostrain per year. This strike-slip motion, combined with compression, confirms a tectonic setting where internal parts of the range are being pulled apart while the edges are being pushed together.

These measurements alone would be enough to warrant a reevaluation of seismic hazard across the Western Alps. However, the study went further by modeling the region as a system of rigid tectonic blocks. Using fault geometry and velocity data, the researchers constructed a three-block model in which the space between HDF and BDF behaves as a cohesive, internally stable unit. This region of low seismicity, despite being flanked by two active faults, could act as a stress bridge, transmitting tectonic forces laterally across the arc.

One of the more unsettling implications of the study is that seismic risk in this area may be concentrated on faults that are quietly accumulating stress while showing little to no surface expression. Unlike regions with frequent small earthquakes that may serve to gradually release strain, faults like the HDF and BDF appear to be capable of holding significant deformation over time. When the eventual rupture occurs, the energy release could be disproportionate to the visible signs leading up to it.

The GNSS data showed vertical motion as well. Uplift rates of up to two millimeters per year have been observed in the high-elevation massifs such as Mont Blanc and Vanoise. This vertical deformation is likely a result of post-glacial rebound, deep mantle processes, and erosional unloading, all of which contribute to the mechanical complexity of the region. While this study did not directly model vertical forces, the authors note that such forces likely play a significant role in driving the horizontal extension observed in the internal Alpine zones.

Notably, the extension in the Briançonnais arc, which includes the HDF, has previously been linked to buoyancy forces acting on the crust and mantle beneath the Alps. These forces may now be interacting with the lateral tectonic pressure driven by the counterclockwise rotation of the Adriatic Plate to the southeast. This interaction of vertical and horizontal drivers appears to be responsible for the simultaneous extension along the HDF and compression along the BDF.

The authors’ analysis of seismic data from the SISMalp regional network confirms the geodetic findings. Earthquakes along the HDF are consistent with normal faulting, reflecting extension, while the BDF is associated with right-lateral strike-slip motion. In both cases, the geodetic strain rates match well with the seismic focal mechanisms. This coherence adds a level of confidence rarely achieved in tectonic studies of slowly deforming regions.

Another layer of concern arises from the fact that these fault zones are located near or beneath populated areas and infrastructure corridors. The Briançonnais region, served by critical transport links, hydropower infrastructure, and Alpine tourism routes, sits directly on the most active segment of the HDF. The BDF, meanwhile, runs along the edge of the Belledonne massif near the Grenoble metropolitan area. Although current slip rates may appear low, even moderate earthquakes in these zones could be damaging due to the structural vulnerability of buildings and the geological amplification effects present in valley floors.

The study also addresses the persistent seismic quiet zones that lie between the two faults. These regions have often been interpreted as tectonically inactive. However, the rigid block model used in this analysis suggests a different interpretation: these zones may in fact be transferring stress from one fault system to the other, operating as mechanical buffers. This setup implies that their seismic silence could be misleading. Rather than being safely inactive, they may represent locked zones storing strain for future release.

Fault locking is a critical consideration in seismic hazard assessment. A fault that slips freely may release energy gradually through small, frequent earthquakes. In contrast, a locked fault builds up elastic strain until it eventually breaks. The researchers explored different fault locking depths in their model, ranging from completely unlocked to locked down to 20 kilometers. In all tested configurations, the model was able to explain roughly 29 percent of the observed velocity field. This partial fit is significant. It suggests that while a large portion of the deformation is accommodated by the known faults, the remainder could be due to distributed strain, unknown fault systems, or vertical processes not captured by horizontal GNSS data.

The study’s implications extend beyond regional geology. It demonstrates how long-term GNSS monitoring can detect deformation in slowly evolving tectonic regions. The Western Alps, far from being geologically quiet, are actively reshaping themselves in ways that current seismic hazard maps may not fully account for. Furthermore, it shows that moderate deformation rates are not necessarily benign. Even small annual movements can lead to dangerous strain accumulations over geological timescales.

It is also clear that the internal and external fault systems of the Western Alps are not acting in isolation. The combination of strike-slip and extensional or compressional forces observed across the HDF and BDF points to a larger tectonic regime influenced by both local and regional drivers. These include the rotation of the Adriatic Plate, residual effects from the last glaciation, mantle dynamics, and the structural legacy of earlier tectonic events. The interaction between all these elements creates a complex and evolving system with the potential for abrupt seismic expression.

Although the paper does not issue predictions about imminent earthquakes, the findings offer strong evidence that the Western Alps should not be considered a low-risk region. Instead, it is an active, slowly deforming orogen whose faults are capable of releasing energy in unpredictable ways. The strain is real, measurable, and unevenly distributed. The faults are connected, not isolated. And the forces acting on this system originate not only from below but also from distant tectonic plates.

As a result of these findings, regional hazard planning may need to revisit the seismic potential of both the High Durance and Belledonne faults. Urban planning, infrastructure design, and emergency preparedness in the affected zones would benefit from integrating these updated deformation rates and fault interaction models. The research also underscores the importance of continued GNSS monitoring, particularly in mountainous regions where visual cues of strain accumulation are absent.

What is perhaps most striking about this study is how slowly the ground beneath us can shift, often without notice or consequence for decades, until it doesn’t. The Western Alps are not inert. Their movement may be glacial in pace, but the consequences of that movement are anything but.

Source Paper:

Walpersdorf, A., Sue, C., Helmstetter, A., et al. (2025). Strain pattern and active faults’ compatibility across the Western Alps revealed by 25 years of GNSS measurements. Preprint posted to Earth and Space Science Open Archive. https://doi.org/10.22541/essoar.175413570.02058342/v1