A record of brittle wedge internal deformation and its link to the subduction earthquake cycle

Abstract:

Geodetic and seismological monitoring of active accretionary margins revealed co- to postseismic forearc extension triggering ≥M_w 6 aftershocks within the hanging wall of subduction zone megathrusts. However, the absolute importance of this process for bulk seismicity within accretionary wedges undergoing severe shortening over millions of years has remained enigmatic. In this study, we infer the conditions of brittle rock failure throughout the formation and growth of the Paleogene accretionary wedge of the central European Alps. We find the predominance of disparate fracture modes within the outer and inner wedge. By combining our dataset with a dynamic Mohr-Coulomb wedge analysis, we show that the observed fracture patterns could only be formed under transient stress fields occurring during co- to postseismic times. We conclude that also over geological timescales wedge internal brittle deformation is linked to the subduction earthquake cycle and seismic hazard within the upper plate is highest following great megathrust earthquakes.