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D. Motion and seismicity through time

Seismicity in the Alpine region is characterized by diffuse, shallow seismicity that only partly reflects Adria-Europe convergence and does not necessarily follow geologically mapped faults. Whereas the domain of highest Adria-Europe convergence at the Alps-Dinarides junction indeed correlates with high rates of seismicity (Fig. 1b), destructive earthquakes have also affected areas where long-term convergence rates are below GPS resolution, for example, in the basement core of the Central Alps in the Simplon area, in the northwestern Alpine foreland near the great Basel earthquake of 1356 (Ustaszewski et al. 2007) or in the northern Dinarides (Ustaszewski et al. 2014). This suggests that factors other than convergence rate affect earthquake activity, for example, stress state, crustal rheology, fluid pressure, and the distribution, orientation and motion history of existing faults. Indeed, rifting and subduction engendered many major faults in the Alps that were reactivated during collision and indentation, and that remain active today, for example, the Giudicarie fold-and-thrust belt (GF in Fig. 2a; Castellarin et al. 2006b, Pomella et al. 2011). Understanding how inherited fault geometry and kinematics interact with today’s stress field is important for assessing seismic hazard.

The junction of the Alps and northern Dinarides is the seismically most active region in the Alps and includes the epicenter of the 1976 Mw 6.5 Friuli earthquake that killed hundreds and left thousands injured and homeless. Yet, seismicity also affects the Alpine forelands, the Eastern Alps south and east of the Tauern Window (1348 Villach earthquake, Reinecker & Lenhardt 1999) and the northern Dinarides (1511 Idrija event, Kastelic et al. 2008). The present stress state in this area reflects ongoing anticlockwise rotation of the Adriatic plate with respect to Europe accommodated by a combination of south-directed thrusting in the eastern part of the Southern Alps (Schönborn 1999, Merlini et al. 2002) and dextral strike-slip faulting in the northern Dinarides (Kastelic et al. 2008). Yet the crustal structures that link the surface trace of these faults with lithospheric mantle slabs at depth remain unknown and are targets of 4D-MB (section V).

Seismicity in the Alpine region has never been monitored homogeneously due to the uneven distribution of permanent stations and different procedures of national seismological services. A unified 3D model for the seismically active zones of the entire Alpine region is not available, and the locations and properties of active faults are poorly known. A thorough analysis of the local seismicity and related structures, as well as cross-border homogenization of location routines need to be undertaken to improve seismic hazard assessment for the region. In 4D-MB, it will be important to distinguish active interseismic faults from aseismic faults in order to constrain the 3D stress field in the Alps. Existing probabilistic seismic hazard maps (GSHAP, SHARE, Faccioli 2013) only marginally account for site effects (amplification and extension of ground motion) especially at long periods in deep sedimentary basins (Bordoni et al. 2012), as well as secondary effects like liquefaction.

Exhumed faults in the Alps offer a rare opportunity to study faulting processes that are otherwise only accessible in rock-deformation experiments (Handy et al. 2007). For example, it has been proposed that the partly exhumed SEMP fault in the Eastern Alps which accommodated Miocene orogen-parallel motion exposes the frozen-in transition from frictional, seismic sliding on discrete fault surfaces to ductile, aseismic creep in shear zones (Frost et al. 2011). In 4D-MB, the integration of microstructural and petrophysical studies with modeling of strain-dependent feedbacks between stress, strain-rate and microstructure promises to yield a better understanding of the factors that control seismicity.