Seismotectonics in the Central Alps: An attempt to link fault structures, seismic activity and recent crustal movements

Owing to still ongoing convergence within the Europe-Adria collision zone, Switzerland is affected by heterogeneously distributed moderate seismic activity. The project SeismoTeCH aims to improve the understanding of the links between the seismic activity, existing fault structures and geodynamics in Switzerland and its close vicinity. We started with compiling existing databases on faults (fault densities, lengths and orientations), seismic activity (spatial hypocenter and magnitude distributions, detection of seismic lineaments, focal mechanisms), orientations of mean principal stress axes and recent crustal movements (GNSS, high precision levelling) in order to establish potential correspondences as well as regional variations.

Due to the long-lasting Alpine deformation, fault-orientation patterns as well as fault-densities vary between specific tectonic domains (Jura/North-Alpine foreland, Alpine frontal sediment nappe systems, External Crystalline Massifs, inner-Alpine domains and Southern Alps). Despite this variability, the fault patterns show first order correlations with the spatial arrangement of newly mapped seismic lineaments, earthquake focal planes and associated focal mechanisms. This correlation indicates a regional geodynamics-controlled reactivation of the specific fault networks during current crustal movements. In terms of recent surface movements, variations in (i) horizontal GNSS movements with respect to stable Europe and (ii) vertical uplift (from levelling and GNSS data) have to be discriminated. (i) From E to W in southern Switzerland (S-Grisons–Ticino–Valais, S of Rhone-Simplon line), horizontal movements change from NW to SW directions (velocities >0.5-0.8mm/yr). The southern Adria crustal block shows minimal to no lateral motions in the W-part and a clear NE-directed motion that is progressively increasing towards the E. This motion can be correlated with the so-called counter-clockwise rotation of the Adriatic plate. North of aforementioned domain, N- to NW-directed movements dominate but velocities decrease progressively from the central Alpine domains (<0.3-0.5mm/yr) towards southern Germany, where they are generally small (<0.3-0.4mm NE-CH). This variability between southern and central/northern Switzerland as well as that from E to W, respectively, is accommodated by NE-SW (Rhone-Simplon system) and N-S oriented strike-slip systems. (ii) Most substantial vertical uplift occurs in a WSW-ENE oriented central Alpine belt ranging from the Valais to the Grisons. Note that absolute values of this vertical uplift are 2-3 times larger compared to horizontal movements in the corresponding domains. Focal mechanisms in this high uplift belt indicate orogen-parallel NE-SW extension mainly in the S-Valais and Grisons accommodated by active normal faulting S of the Penninic front. Uplift rates gradually decrease towards the N- and S-Alpine foreland as well as towards Austria and France. Data even suggest tendencies of subsidence at very low rates in the Bresse graben, Upper Rhine graben as well as somewhat more pronounced ones in the eastern Po-plane but not in the CH-Molasse basin. Parts of the northern Alpine foreland exhibit upper to lower crustal seismic activity, while in the thick-crustal-root-enhanced high uplift domains upper crustal seismicity dominates and earthquakes below 20km depth do not occur.

Overall recent surface movements and seismicity in and along Central Alpine crustal blocks are affected by buoyancy-driven vertical combined with transpressional/-tensional horizontal movements indicating a lithosphere-scale geodynamic forcing.