Effects of variations in density and effective viscosity of the mantle lithosphere on the distribution of intraplate earthquakes in western and central Europe

The distribution of seismicity in intracontinental western and central Europe is not well understood despite evidence for tectonic forces and glacial isostatic adjustments to partially affect local stress and strain relationships. Our region of interest, located between the northern Alpine Deformation Front and the southwestern margin of Fennoscandia, is well differentiated into seismically quiet domains (e.g., most of Ireland, the southern North Sea and the Paris Basin region) and elongated zones of increased seismicity, such as across mainland Britain and the European Cenozoic Rift System. Some inherited zones of crustal weakness have been suggested to control the observed clustering of active deformation, but the majority of earthquakes in the region cannot unequivocally be mapped to specific crustal discontinuities. To investigate potential effects of upper mantle heterogeneities on the lateral distribution of earthquakes across stable western and central Europe, we have derived thermal field variations from a continent-scale tomographic shear-wave velocity model by using a Gibbs's free energy minimization approach. This way we find that seismicity in this intraplate region is largely limited to areas that exhibit a temperature-controlled low-density layer in the uppermost lithospheric mantle and preferentially clustered above large lateral gradients in upper mantle effective viscosity. We propose that the spatial correlations between mantle low-density bodies and crustal seismicity reflect gravitational instabilities due to buoyancy forces within the mantle lithosphere. In addition, lateral contrasts in temperature and related effective viscosity seem to foster localized deformation within the shallow mantle which imposes differential loading of the overlying crust and earthquake clustering.