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Technischer Bericht NTB 84-37

Rezentes Spannungsfeld in der zentralen Nordschweiz

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The regional stress field and its local variation were detected for the northern part of Central Switzerland using an overcoring technique (doorstopper) and the observation of breakouts in deep boreholes. The results were compared with fault plane solutions of earthquakes.

In this area at the south-eastern end of the Rhinegraben, the stress field is very inhomogeneous. The general direction of the mean stress in Central Europe (N140°E±10°) was observed at only one location in the case of the overcoring measurements. All other in-situ (overcoring) measurements are up to 80° off this general direction. These changes of the state of stress near the surface were confirmed by the directions of borehole breakouts in deep wells. The spalling of the borehole wall indicates a stress direction of N160°E±10° within a sediment trough and N7°E±10° in the sediments on the top of the shearplane of the Jura overthrust. The general direction of the European stress field was measured exclusively in the crystalline basement. This is the first time that a rotation of stress direction with depth has been observed directly. The data indicate the mechanical decoupling of the basement and the sediments.

In the area of the Dinkelberg the data of the overcoring measurements show tensile stresses which are in agreement with results of the faultplane solution of normal-fault earthquakes in the area of Wies and Kandern.

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