The General Pattern of Seismotectonic Dislocations in Central Europe as the Background for the Liège Earthquake on November 8, 1983

Abstract

The seismotectonic activity of the Central European area between the Alps and the North Sea is controlled by two essential factors. The first is a wide-spread regional stress field within the Earth’s crust characterized by a rather uniform trend of the maximum compressive stress in NW-SE direction (145° with respect to North). The second controlling factor is the post-Hercynian block mosaic and the existence of deep reaching major fault structures like the Rhine Graben System which form distinct zones of crustal weakness. Under the influence of the large-scale stress field characteristic seismotectonic block movements are going on along pre-existing zones of crustal weakness which can be described by a rather simple deformation model.

The focal mechanism of the Liege earthquake on November 8, 1983, fits well to the large-scale deformation model. The fault plane solution elaborated on the basis of P wave first motions of 65 seismic stations in Central Europe reveals a prevailing strike-slip dislocation combined with a small thrust component. The fault movement is either right-lateral along a WSW-ENE trending plane, or left-lateral along a NNW-SSE trending plane. From geological evidence it is assumed that the WSW-ENE plane is the true rupture plane. The pressure axis is orientated NW-SE and nearly horizontal.

The unusual high stress drop of the Liège mainshock of approximately 156 bar as estimated from S wave spectra points to comparatively high crustal stresses at shallow depth in the northwestern border region of the Rhenish Massif. This is in agreement with the results of a hydraulic fracturing experiment in the research borehole Konzen 1 (near Monschau, 50 km SE of Liege), where a maximum shear stress of 38 bar has been found at a depth of 400 m with increasing tendency to greater depth.