Dehydration-induced earthquakes identified in a subducted oceanic slab beneath Vrancea, Romania

Abstract

Vrancea, Eastern Romania, presents a significant intermediate-depth seismicity, between 60 and 170 km depth, i.e. pressures from 2 to 6.5 GPa. A debate has been lasting for decades regarding the nature of the seismic volume, which could correspond to the remnant of a subducted slab of Tethyan lithosphere or a delamination of the Carpathians lithosphere. Here we compile the entire seismicity dataset (≈ 10,000 events with 2 ≤ Mw ≤ 7.9) beneath Vrancea for P > 0.55 GPa (> 20 km) since 1940 and estimate the pressure and temperature associated with each hypocenter. We infer the pressure and temperature, respectively, from a depth-pressure conversion and from the most recent tomography-based thermal model. Pressure–temperature diagrams show to what extent these hypocentral conditions match the thermodynamic stability limits for minerals typical of the uppermost mantle, oceanic crust and lower continental crust. The stability limits of lawsonite, chloritoid, serpentine and talc minerals show particularly good correlations. Overall, the destabilization of both mantle and crustal minerals could participate in explaining the observed seismicity, but mantle minerals appear more likely with more convincing correlations. Most hypocentral conditions match relatively well antigorite dehydration between 2 and 4.5 GPa; at higher pressures, the dehydration of the 10-Å phase provides the best fit. We demonstrate that the Vrancea intermediate-depth seismicity is evidence of the current dehydration of an oceanic slab beneath Romania. Our results are consistent with a recent rollback of a W-dipping oceanic slab, whose current location is explained by limited delamination of the continental Moesian lithosphere between the Tethyan suture zone and Vrancea.

Horizontal Force Required for Subduction Initiation at Passive Margins With Constraints From Slab Detachment

“…In the Vrancea area (e.g., Wenzel et al, 1998) a nearly vertically hanging slab which is still attached to the overlying plate induces earthquakes at an intermediate depth between 70 and 180 km. However, the maximum depth of the slab might be much deeper (Sperner et al, 2001;Ferrand and Manea, 2021). Also, recent seismic data from the Hindu Kush indicates a potential onset of slab detachment or delamination at depths of ca.…”

Raspberry Shake-Based Rapid Structural Identification of Existing Buildings Subject to Earthquake Ground Motion: The Case Study of Bucharest

“…Romania is an active seismic country and Bucharest is one of the most earthquakeendangered capitals in Europe [51,52], threatened mostly by the earthquakes in the Vrancea intermediate-depth seismic source [33,53]. In every century, one to five strong earthquakes (M W > 7.0) have occurred in Romania [33], with the most recent destructive ones being in 1940 and 1977.…”

A switch from horizontal compression to vertical extension in the Vrancea slab explained by the volume reduction of serpentine dehydration

“…4 – 8 ). According to most recent findings, the latter would correspond to ruptures occurring within a remnant slab of subducted lithospheric mantle, whose location is explained by slab retreat and limited continental delamination during the late Neogene 9 . Moderate crustal seismicity is recorded all over the Carpathian region, but the far more intense and persistent activity occurs in a small subcrustal seismogenic volume beneath the SE bend of the Carpathian Arc.…”

“…Some studies argue for a gravitational instability 5 , 13 . The debate on the nature and origin of the Vrancea seismic body has recently been detailed in the form of a transdisciplinary review 9 .…”

“…Knowing about the stress state and orientation of the stress field within the slab could help grasp the mechanisms involved. Therefore, a better understanding of the link, if any, between triggering mechanisms 9 and focal mechanisms 22 , 23 of the seismic ruptures is needed.…”