Non-tidal gravity change and Vrancea intermediate-depth seismicity

Right in the SE bend of East Carpathians, in the so-called Vrancea zone, unusual intermediate-depth seismicity (70-200 km) occurs within full intra-continental environment.
Despite many years of national and international research efforts its genesis and mechanisms are still subject to debate. Seismic tomography studies have revealed the presence of a high velocity body bellow the crust in the area, and most hypotheses were based on the assumption of the existence of a paleo-subduction zone, with earthquakes foci located in a hanging slab relict. However, during the last years, new, alternate models have been proposed (e.g. lithosphere delamination, gravitational instability or the unstable triple junction) in order to explain some strange peculiarities in the earthquakes distribution and some associated phenomena (e.g. crust deformation).
In order to study the geodynamic processes in the area, the Solid Earth Dynamics Department in the Institute of Geodynamics of the Romanian Academy has designed and implemented a special infrastructure in the area. The dedicated Vrancea geodetic network (VGN) is aimed at monitoring crust deformation and space-time gravity change within epicentre area of the intermediate earthquakes and surrounding region. Especially designed steel-reinforced concrete pillars (0.5 m x 0.5 m x 2 m), weighting more than 1500 kg are implemented in each base-station of VGN to offer a stable environment for high accuracy repeated gravity and GPS observations within each base station of the network.
Following several years of repeated field campaigns some intriguing aspects have been revealed.
While the epicentre area of the intermediate earthquakes experiences a slight relative subsidence as compared to the rest of East Carpathians catena (generally uprising several millimetres per year due to denudation and erosion), the gravity field seems to continuously decrease. The gravity lowering becomes obvious after each significant earthquake advocating for an ongoing causally interlinked process.
Attempts to explain the strange association of topography subsidence with gravity decrease led to the idea of the crust vertical stretching caused by the eclogitization of its lower part penetrating the upper mantle.
Numerical simulation of the non-tidal gravity change has showed a yearly rate of 4-6 mm/year vertical stretching of the crust, in agreement with information previously provided by seismic end tectonic studies.
Some speculations concerning the geodynamic scenarios for Vrancea active seismic zone are finally presented and discussed in the paper.

Acknowledgement. The research has been partly supported through the grant POS CCE O 2.1.2. ID 593 (contract 182/2010).