Present-day stress field pattern in the Vrancea seismic zone (Romania) deduced from earthquake focal mechanism inversion

Abstract

Vrancea seismogenic zone in the South-Eastern Carpathians is characterized by localized intermediate-depth seismicity. Due to its complex geodynamics and large strain release, Vrancea represents a key element in the Carpatho-Pannonian system. Data from a recently compiled catalogue of fault plane solutions (REFMC) are inverted to evaluate stress regime in Vrancea on depth. A single predominant downdip extensive regime is obtained in all considered clusters, including the crustal layers located above the Vrancea slab. The prevalent stress regime confirms previous investigations and requires some mantle-crust coupling. The S3 principal stress is close to vertical, while S1 and S2 are horizontal, oriented perpendicularly and respectively tangentially to the Carpathians Arc bend. This configuration is present at any depth level. According to seismicity patterns, there are two main active segments in the Vrancea intermediate-depth domain, at 55 – 105 km and 105 – 180 km, both able to generate major events. The configuration of the tectonic stresses as resulted from inversion is similar in both segments. Also, high fault instability (I > 0.95) is characterizing the segments. The only notable difference is given by the friction and stress ratio parameters which drop down in the bottom segment from μ = 0.95 to μ = 0.55 and from R = 0.51 to R = 0.29. This variation is attributed to possible weakening processes activated below 100 km depth and can explain the intensification of seismicity production as earthquake rate and average energy release in the lower segment versus the upper segment.

What we know and what we don't know about the earthquakes in the Vrancea region (Romania)

“…If the rupture process for major earthquakes can be produced only where background seismicity is generated, then the limitation of the focal mechanisms with the rupture plane oriented across Carpathians arc bend to events with magnitude below 6.5 could be explained by the geometrical limitation of the seismicity in this direction (~ 30 km), while the rupture process for the largest earthquakes can be deployed only on a perpendicular plane (NE-SW oriented) since the seismicity dimension on this direction allows the rupture extensions up to ~ 70 km. [5] analysed the stress regime on depth in the Vrancea region by inverting the fault plane solutions. They found a predominant downdip extensive regime in the entire seismogenic volume in the upper mantle, as well as in the crustal layers located above the Vrancea slab.…”