The moderate-size earthquake (ML 5.7) which occurred on November 22, 2014 in Vrancea region (Romania), is the largest crustal event instrumentally recorded. Its aftershock sequence lasted around 70 days, 222 earthquakes with M L ≥ 0.1 being located using the records collected by the Romanian seismic network. The seismic sequence occurred mainly in the lower crust (depths greater than 25 km), and the epicenter distribution-along a NNE-SSW direction-follows the orientation of the Vrancea crustal earthquakes alignment. The spatio-temporal distribution of the seismic activity, as well as the seismic energy release during the seismic sequence are analysed in detail, and the focal mechanisms of the largest events-34 shocks with local magnitude ≥ 1.8-are determined using reliable P-wave polarities and amplitude ratios. Taking into consideration that the moderate-size shock on November 22, 2014 is the strongest instrumentally recorded crustal earthquake in the region, its focal mechanism provides highly relevant seismological information on the deformation field in front of the Carpathian bend. The obtained fault plane solution indicates normal faulting with a dominant dip-slip component; both nodal planes being oriented NW-SE.
The Vrancea seismic zone, located in the bend region of the South-Eastern Carpathians, is a unique area with both crustal and intermediate-depth seismic activity and is known as one of the most active seismic area in Europe. Moderate crustal seismicity is recorded all over the Carpathian region, but the far more intense activity occurs in a small subcrustal seismogenic volume beneath the SE‐bend of the Carpathian arc with about 20 × 50 km lateral and 110 km vertical extent (70–180 km depth). A unique slab geometry, likely preserved until the present, causes stress localization due to the slab bending and subsequent stress release resulting in large mantle earthquakes in the region. The main focus of this study is to determine the focal mechanisms for events with a magnitude larger than 2.7, between 2005 and 2020 and evaluate the current stress field along the Vrancea subcrustal region, from the derived fault plane solutions. The main style of faulting for Vrancea subcrustal events presents a predominant reverse one, with two main earthquakes categories: the first one with the nodal planes, oriented NE–SW parallel with the Carpathian Arc and the second one with the nodal planes, oriented NW–SE perpendicular on the Carpathian Arc. The results of stress inversion indicate a dominant thrust faulting style, with an average stress regime index of 2.87. The stress pattern shows similar partitioning with vertical extension in the slab and no preferred orientation in the overlying crust, showing a transition regime from the extensional regime in the Moesian Platform to the compressional regime in the Vrancea subcrustal zone.
<p>Seismic activity in Romania encounters a variety of tectonic domains, from crustal earthquakes, which occur along active faults, to intermediate-depth seismicity that extends down to 200 km depth and which is limited in a small subcrustal seismogenic volume beneath the SE bend of the Carpathians arc. Crustal depth seismicity in Romania is distributed throughout the territory, the areas with important seismic activity and which are also analyzed in this paper being: Vrancea, Fagaras-Campulung, Banat, Dobrogea zones.</p> <p>Using the polarity of first arrivals methodology (FOCMEC software developed by Havskov et. al 2020), we calculate the focal mechanisms for crustal depth earthquakes that occurred in these areas between 2012 and 2022. We then derive the regional distribution of the stress field through a linear inversion using the focal mechanisms obtained in this study, supplemented by the solutions of the REFMC catalog (Radulian et al 2020). Inversion results vary from the compressive regime in the SE Carpathians bend zone, to strike-slip regime in Banat zone, and extensive regime in Dobrogea area. The stress field configuration is matching generally the configuration of the global stress pattern as shown by the World stress map with the exception of some significant deviations that reflect local conditions.&#160;</p> <p>The analysis of focal mechanisms as well as the stress field provides a basis for transdisciplinary discussions and collaborations between researchers from various fields.</p>
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