REGIONAL PATTERNS OF TECTONIC STRESS IN EUROPE

Nearly 1500 stress orientation determinations are now available for Europe. The data come from earthquake focal mechanisms, overcoring measurements, well bore breakouts, hydraulic fracturing measurements, and young fault slip studies and sample the stress field from the surface to seismogenic depths. Three distinct regional patterns of maximum compressive horizontal stress (S(Hmax) orientation can be defined from these data: a consistent NW to NNW S(Hmax) stress orientation in western Europe; a WNW-ESE S(Hmax) orientation in Scandinavia, similar to western Europe but with a larger variability Of S(Hmax) orientations; and a consistent E-W S(Hmax) Orientation and N-S extension in the Aegean Sea and western Anatolia. The different stress fields can be attributed to plate-driving forces acting on the boundaries of the Eurasian plate, locally modified by lithospheric properties in different regions. On average, the orientation of maximum stress in western Europe is subparallel to the direction of relative plate motion between Africa and Europe and is rotated 17-degrees clockwise from the direction of absolute plate motion. The uniformly oriented stress field in western Europe coincides with thin to medium lithospheric thickness (approximately 50-90 km) and high heat flow values (>80 m W/M2). In western Europe a predominance of strike-slip focal mechanisms implies that the intermediate principal stress is vertical. The more irregular horizontal stress orientations in Scandinavia coincide with thick continental lithosphere (I 10-170 km) and low heat flow (<50 M W/M 2). The cold thick lithosphere in this region may result in lower mean stresses associated with far-field tectonic forces and allow the stress field to be more easily perturbed by local effects such as deglaciation flexure and topography. The stress field of the Aegean Sea and western Anatolia is consistent with N-S extension in a back arc setting behind the Hellenic trench subduction zone. The stress field is influenced in places by regional geologic structures, e.g., in the Western Alps, where S(Hmax) directions show a slight tendency toward a radial stress pattern. Not all major geologic structures, however, appear to affect the S(Hmax) orientation, e.g., in the vicinity of the Rhine rift system horizontal stress orientations are continuous.