Earthquake focal mechanisms and stress orientations in the eastern Swiss Alps

This study presents an updated set of earthquake focal mechanisms in the Helvetic and Penninic/Austroalpine domains of the eastern Swiss Alps. In eight cases, based on high-precision relative hypocentre locations of events within individual earthquake sequences, it was possible to identify the active fault plane. Whereas the focal mechanisms in the Helvetic domain are mostly strike-slip, the Penninic/Austroalpine domain is dominated by normal-faulting mechanisms. Given this systematic difference in faulting style, an inversion for the stress field was performed separately for the two regions. The stress field in the Penninic/Austroalpine domain is characterized by extension oriented obliquely to the E-W strike of the orogen. Hence, the Penninic nappes, which were emplaced as large-scale compressional structures during the Alpine orogenesis, are now deforming in an extensional mode. This contrasts with the more compressional strike-slip regime in the Helvetic domain towards the northern Alpine front. Relative to the regional stress field seen in the northern Alpine foreland with a NNW-SSE compression and an ENE-WSW extension, the orientation of the least compressive stress in the Penninic/Austroalpine domain is rotated counter-clockwise by about 40A degrees. Following earlier studies, the observed rotation of the orientation of the least compressive stress in the Penninic/Austroalpine region can be explained as the superposition of the regional stress field of the northern foreland and a uniaxial extensional stress perpendicular to the local trend of the Alpine mountain belt.