A possible slow-slip-event in the Vrancea seismic active region of Romania

In the last 300 years the window of time for two consecutive large and destructive intermediate-depth earthquakes in Vrancea (Romania) was between 36 and 102 years. An explanation for the larger window of time might be a release of stress produced by a slow-slip-event (SSE). In a vertical sinking slab slightly attached from the Earth’s crust both large earthquakes and SSE are expected to generate a downward movement in the vertical displacements of GPS data. The building-up of stress in the asperity preventing a steady aseismic sinking was expected to be transmitted upwards to faults in the crust and recorded based on a magnetotelluric phase splitting effect. A large stress build-up has been suggested around a fault in the years 2012–2013, but no large earthquake was recorded. We supposed a large SSE in the year 2013–2014 with a duration of 13 months released the accumulated stress. GPS stations in the epicentral region of Vrancea seismic active region supported our suggestion by showing a downward displacement of vertical data obtained for the year 2014. However, the vertical displacements are small and other possible causes than SSE need to be taken into account.