Stress field in the Northern Rhine area, Central Europe, from earthquake fault plane solutions

Abstract

Fault plane solutions (FPS) from 110 earthquakes in the northern Rhine area with local magnitudes, ranging from 1.0 to 6.1, and occurring between 1976 and 2002 are determined. FPS are retrieved from P-wave first motions using a grid search approach allowing a detailed exploration of the parameter space. The influence of the 1D velocity model on take-off angles and resulting FPS is examined. All events were relocated with a recently developed minimum 1D model of the velocity structure [J. Geophys. Res. (2003)]. Rose diagrams of the orientation of P, T and B axes show a clear preference of trends of P and T axes at N292°E and N27°E, respectively. The majority of B axes trend in northerly directions. Plunges of P and T axes are mostly around 45° while most B axes are subhorizontal. The main direction of the maximum horizontal stress directly inferred from the fault plane solutions is N118°E. To calculate the orientations of the principal stress axes and the shape of the stress tensor, the inversion method of Gephard and Forsyth [J. Geophys. Res. 89 (1984) 9305] was applied to the whole data set and to several subsets of data. The subsets were formed by grouping events from various geological and tectonic areas and by grouping events into different depth ranges. The subset areas include the Lower Rhine Embayment, the Rhenish Massif, the middle Rhine area, the Neuwied Basin and the area known as the Stavelot–Venn Massif. Inversion of the entire data set shows some ambiguity between a strike-slip and extensional stress regime, with a vertical axis for the medium principal stress and a trend of N305°E and N35°E for the σ 1 and σ 3 axis, respectively, as the best fitting tensor. Earthquakes from the Lower Rhine Embayment and, to some degree, from the middle Rhine area indicate an extensional stress regime. In the Lower Rhine Embayment, plunge and trend of the σ 1 axis are 76° and N162°E and for the σ 3 axis 7° and N42°E. The best fitting solution for the area of the Stavelot–Venn Massif is a strike-slip regime with subhorizontal σ 1 and σ 3 axes with a trend of N316°E and N225°E, respectively. Stress orientations found here agree overall with the results from earlier studies based on smaller data sets. The directions of the maximum and minimum horizontal stresses inverted from focal mechanisms agree well with the stress field predicted by the European Stress Map. This confirms earlier interpretations that the stress field of the Rhine Graben system is controlled by plate driving forces acting on the plate boundaries. However, amplitudes of the stresses change on a local scale and with depth. Estimates of the absolute magnitude of principal stresses favor a normal faulting regime in the shallow crust (above 12-km depth) and a strike-slip regime in the lower crust.