Tectonic stresses in the Vrancea seismic zone

The scientists from the Netherlands, Russia, and Romania estimate earthquake source parameters in Vrancea seismic zone and surrounding regions between 2014 and 2020, and determine the stress field via focal mechanism inversion and unsupervised machine learning. The work was published in Tectonophysics (Q1, JIF=3.048).

Seismic-tomographic image of the Vrancea slab and hypocenters of earthquakes (circles and asterisks indicate the location and magnitude of seismic events). The top surface illustrates the topography. The blue surface represents the isosurface of 3% positive anomalies of P-wave velocity obtained via teleseismic data inversion. Focal spheres are fault-plane solutions for the four largest Vrancea intermediate-depth earthquakes in the XXth century. The right panel presents the horizontal slice of the seismic-tomographic image at depth 100 km.

Vrancea seismic zone, located at the South-Easter Carpathians bend, releases the largest seismic energy in continental Europe. Strong earthquakes at depths of 80-180 km in Vrancea shook cities and other settlements at a distance of several hundred kilometers from the epicenters of these events, caused destruction in Bucharest, and were felt in Moscow at a distance of more than 1,700 km. Current research is aimed to better understanding earthquake generation and its relation with lithospheric deformation.

It is shown that the maximum horizontal stress in the earth’s crust is consistent with the kinematics of surface faults and geodetic data on the horizontal velocities of the plates relative to Eurasia. Inside a seismogenic body located at mantle depths, vertical compressive stresses dominate. However, the stress coefficients obtained are small: about 0.2 for mantle earthquakes with a magnitude of Mw > 4 and about 0.4 for earthquakes with Mw < 4, which challenges the assumption of brittle fracture. It has been shown that increased pore fluid pressure reduces stress coefficients, implying that dehydration embrittlement may contribute to the occurrence of seismicity at depths of 80-180 km in the Vrancea plate. A comparison with seismic tomography and anisotropy studies shows a high correlation between the directions of maximum horizontal stresses and seismic anisotropy, especially below depths of 130 km, suggesting that mantle flows resulting from the sinking of the ancient plate may also contribute to the accumulation of stresses in the plate and the seismic potential. “The current data and models presented in this paper on the stress state in Vrancea have once again confirmed our hypothesis that regional stresses and seismicity are associated with dehydration embrittlement and the influence of mantle flows, proposed earlier in [Ismail-Zadeh et al. (PAGEOPH, 2000) and Ismail-Zadeh et al.(PEPI, 2005)] “,- says Doctor of Physical and Mathematical Sciences A. T. Ismail-Zadeh, chief researcher of the Institute of Earthquake Prediction Theory and Mathematical Geophysics of the Russian Academy of Sciences and co-author of the article.

Source: Petrescu L., Borleanu F., Radulian M., Ismail-Zadeh A., Matenco L. Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings// Tectonophysics. 2021.V. 799.Article 228688. DOI:10.1016/j.tecto.2020.228688.