Tectonic stress, seismicity, and seismic hazard in the southeastern Carpathians

Abstract

Intermediate-depth earthquakes in the Vrancea region occur in response to stress generation due to descending lithosphere beneath the southeastern Carpathians. In this article, tectonic stress and seismicity are analyzed in the region on the basis of a vast body of observations. We show a correlation between the location of intermediate-depth earthquakes and the predicted localization of maximum shear stress in the lithosphere. A probabilistic seismic hazard assessment (PSHA) for the region is presented in terms of various ground motion parameters on the utilization of Fourier amplitude spectra used in engineering practice and risk assessment (peak ground acceleration, response spectra amplitude, and seismic intensity). We review the PSHA carried out in the region, and present new PSHA results for the eastern and southern parts of Romania. Our seismic hazard assessment is based on the information about the features of earthquake ground motion excitation, seismic wave propagation (attenuation), and site effect in the region. Spectral models and characteristics of site-response on earthquake ground motions are obtained from the regional ground motion data including several hundred records of small and large earthquakes. Results of the probabilistic seismic hazard assessment are consistent with the features of observed earthquake effects in the southeastern Carpathians and show that geological factors play an important part in the distribution of the earthquake ground motion parameters.

Ground-motion prediction equations for the intermediate depth Vrancea (Romania) earthquakes

“…Note that the earthquake sources were described as the point sources and no specific peculiarities of the radiation pattern and the rupture propagation were considered. Table 4 compares results of the modeling for the epicientral area (Focsani basin) and the Bucharest area with the available observed data and the results of site-dependent probabilistic seismic hazard assessment (Ismail-Zade et al 2007;Sokolov et al 2004a). Obviously, it is not possible to expect a perfect agreement between the available intensity observations and the results of the modelling; however the general features of the earthquake effect are reproduced quite adequately.…”

“…(a) the earthquake of November 10, 1940 (Mw 7.7, depth 140 km); (b) the earthquakes of March 4, 1977 (M W 7.4, depth 95 km)Table 4Values of ground motion parameters, which were observed (in parentheses) and modelled for large and upper limits of the modelled parameters were estimated using the AM and AM1 variant of attenuation relationships. The results of site-dependent probabilistic seismic hazard assessment(Ismail-Zade et al 2007;Sokolov et al 2004a) are shown for comparison. a the earthquake of 1977 was recorded in Bucharest by one station (INC).…”

Tectonic regimes and stress patterns in the Vrancea Seismic Zone: Insights into intermediate-depth earthquake nests in locked collisional settings

“…Despite its remote location from the most active European collisional boundaries, the Vrancea earthquakes pose a significant seismic risk for the densely populated regions of Romania and neighbouring countries (e.g. Ismail-Zadeh et al, 2007;Sokolov et al, 2009). Seismic tomographic images reveal positive anomalies of seismic wave velocities in the upper-mantle beneath the southeastern (SE) Carpathians and the foreland region extending to depths of ~400 km (Wortel and Spakman, 2000;Martin et al, 2006;Koulakov et al, 2010;Ren et al, 2012;Zhu et al, 2015;Baron and Morelli, 2017).…”

Factor Analysis in Seismology

“…The development of a more complex and adequate attenuation model is the subject of study by many seismologists. For example, an analysis of the macroseismic and instrumental data of earthquakes of the intermediate depth of the Vrancea locus revealed specific features of the effects of earthquakes: the impact on large areas with a predominant orientation of NE-SW; a large degree of dependence of the seismic displacement amplitude of the soil on local and regional geological conditions, as compared with the magnitude and distance from the source; large variability of parameters of strong ground movements; the reflection of the topography of the earth's surface by isolines [Ismail-Zadeh et al, 2007]. In the near zone, where r ~ h, (r is the epicentral distance, h is the depth of the earthquake source), the geometry of the earthquake source has a decisive influence on the configuration of the macroseismic field [Shebalin, 1961;1980].…”