The purpose of this work is to investigate how the local soil conditions in the Bucharest area control the ground motion characteristics using all the available acceleration data for large and moderate size Vrancea earthquakes. The proposed comparative analysis shows no systematic variations from one site to the other in agreement with the relatively uniform subsurface structure beneath the city. The main conclusion of this study is that for the particular case of Bucharest and Vrancea earthquakes, basically the microzonation is emphasizing no areas with significant variation in the amplification factor. At the same time, the source radiation is clearly controlling the seismic response depending on the magnitude. Thus, for the largest shocks (Mw ≯ 7), the seismic response in the range of 1-2 s period is exceptionally enhanced leading to the disastrous damage noticed for the high-tall buildings in the city.
Notable clustering effects in seismic activity are observed at the sharp bend of the South-Eastern Carpathians both at subcrustal and crustal domains. The seismic energy release is strongly increasing with depth, where apparently the principal source controlling the geodynamics of the entire system is located. New developments of instrumental monitoring and relatively extended historical data spanning several seismic cycles in Vrancea allow refined analyses of earthquake patterns, outlining high-resolution images of specific alignments of seismicity in the Vrancea region. Refined joint locations are carried out for the most recent earthquakes, produced after 1995. In parallel, a new approach has been implemented to produce numerical simulations of the Vrancea seismic cycle and to reproduce its main characteristics: return period, main shock magnitude, depth location, non-linear frequency-magnitude distribution, aftershock activity and their variability from one cycle to other.
The Vrancea seismic region represents a unique case of well-defined and intense intermediate-depth earthquake activity as a consequence of specific geodynamic processes at the continental contact between East-European, Moesian and Intra-Alpine plates. Apart the subcrustal earthquakes generated in the Vrancea slab, the analysis of seismicity puts into evidence two other clusters of subcrustal earthquakes (
> 50 km) toward the back-arc side of the SE Carpathians bend, one to the west (Sinaia), the other to the north-west (Braşov Depression) relative to the Vrancea seismic source. The hypocenters lay down between 50 to 105 km in Sinaia and between 50 and 136 km in Braşov Depression. The rate of seismic energy release is much lower (
≤ 3.7 in Sinaia and
≤ 3.1 in Braşov Depression) than for Vrancea activity. The seismicity pattern in the SE Carpathians back-arc region upper mantle correlates well with the high-velocity structures depicted by seismic tomography investigations suggesting possible remnant deep lithospheric roots apart from the narrow well-defined slab generating Vrancea major earthquakes. The results outline significant lateral heterogeneities in the mantle and provide new data for incorporating seismological, geotectonic and volcanological data in a unified modeling of the complex processes taking place in the study region.
The Vrancea region, in the South-Eastern Carpathians (Romania), represents a unique case among the seismic areas in the world taking into account the extreme concentration and persistence of seismicity and tectonic stress field. Our goal is to show that the particular shape of attenuation and shear wave splitting properties can be interpreted in the light of the decoupling and slab retreat processes, which hypothetically induce a specific configuration of the upper mantle flow. Delamination and break-off processes combined with retrograde motion of the slab imply lateral asymmetry in flow geometry and geotectonic properties. Particularly relevant is the strike-parallel flow localized in front of the downgoing slab, in contrast with the steeply dip flow along the slab in the back side. The local upper mantle flows around the descending seismic active body explains the contrast of the seismic wave attenuation in the back-arc region against fore-arc region and the strong anisotropy anomaly observed in the South-East Carpathians (time delays of 1.5–2 s). Outside the epicentral area, the coherent pattern of the shear wave splitting follows the trends of the deformation field as outlined by the GPS measurements, in favour of a strong coupling between the surface and mantle processes.
We present a short review of the most recent results coming from the numerical modeling of seismic hazard and interpretation of the new observations provided by the recently installed seismic stations in Romania. There are still controversial questions related to the asymmetric distribution of the ground motion radiated by the Vrancea intermediate-depth earthquakes and which is the main factor responsible for this particular distribution: seismic source, structural model, site effects or vulnerability. Our main goal is to provide a solution to this key problem, with direct implications upon the seismic hazard assessment. NE-SW elongation of the isoseismals and the maximum values in the Romanian Plain are well explained by the source radiation pattern and the average structural model. The attenuation toward NW is shown to be a frequency-dependent effect, much more important in the high frequency range (≯1 Hz). We conclude that the present seismic hazard of Romania, computed by the deterministic approach, fits well, as a first approximation, the ground motion distribution for the low-frequency band, and the apparent contradiction with the historically-based intensity maps arises mainly from a systematic difference in the vulnerability of the buildings in the intra- and extra-Carpathians regions.
The seismic hazard assessment in dense-populated areas and the strategic objectives design are based on the study of seismicity of the seismogenic sources (which can have an effect at the site above a specified minimum level that is considered risky) and the attenuation between focus and site. The purpose of this paper is to provide a complete set of information required for a probabilistic assessment of the seismic hazard in the southeast Romania relative to the following seismic sources: Vrancea zone (in the crust and intermediate-depth domains), Predobrogean depression zone, Intramoesian zone and Shabla-Dulovo (Bulgaria) zone. As an example, the obtained input parameters are used to evaluate the seismic hazard distribution due to the crustal earthquakes applying the McGuire's procedure (1976). These preliminary results are in good agreement with the previous research based on deterministic approach (Radulian et al. 2000).
Attenuation specific from Vrancea intermediate-depth focus toward SE is revised at length in order to re-evaluate the seismic hazard at the nuclear power plant (NPP) Cernavoda. To this aim, we took into consideration all the strong motion data available for the region and test a large range of relationship types. Since the instrumental data are limited as magnitude and depth intervals, the extrapolation based on the attenuation laws deduced from these data to great magnitudes is very instable and must be carefully considered. The extrapolation outside the characteristic domain of the available database leads to large deviations, especially at large magnitudes and for the relations which contain the depth as independent parameter.