Attached or not attached—evidence from crustal stress observations for a weak coupling of the Vrancea slab in Romania

Abstract

The crustal stress pattern of Romania provides key insights into whether the Vrancea slab with its seismogenic volume between 70 and 175 km depth is still coupled to the crust and thus acts as a stress guide, or whether it is already in a state of detachment from the crust. Knowledge of the state of the slab under Vrancea is particularly critical because the slab attached to the crust can result in future strong earthquake occurrence in the crust and even in the currently aseismic zone between 40 km and 70 km depth, potentially causing severe damage. Our analysis of the contemporary tectonic stress observations in the context of potential stress sources and the comparison with numerical modelling shows that the crustal stress pattern in Romania is heterogeneous and does not contain a long wave-length stress pattern that would be expected if there is a strong present-day coupling between the subducted slab and the upper plate, or if lateral plate boundary forces would control the regional stress pattern. Therefore, we conclude that the crustal stress pattern of Romania is characterised by small differential horizontal stresses where local stress sources (third-order effects) are responsible for the observed heterogeneity of stress orientations and that the subducted slab under Vrancea is only weakly coupled to the crust.