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  • Author or Editor: I Paskaleva x
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The capital of Bulgaria, Sofia is a growing city with population around 1.22 million. The city is exposed to a high seismic risk since it is placed in the centre of Sofia seismic area. Over the centuries in the town of Sofia the macroseismic intensities have been larger than IX (MSK). A study of the site effects and the microzonation of a part of metropolitan Sofia, based on a  modelling of seismic ground motion along three cross sections are presented. Realistic synthetic strong motion waveforms have been computed for an expected scenario earthquake (M=7) applying a hybrid modelling method, based  on the modal summation technique and finite differences scheme. The site amplification is determined in terms of response spectra ratio (RSR). A set of time histories and quantities of earthquake engineering interest are supplied, that allow the definition of six zones characterized by specific response spectra. The results from this study constitute a “database” that describes the ground shaking of the urban area.  The synthetic velocigrams are employed to calculate the distribution of the horizontal strain factor Log10e using a simplified relation between particle velocity and velocity of shear waves in the surface layer. It is shown  that it is possible to estimate liquefaction susceptibility in terms of standard penetration tests (SPT), Nvalues and initial over burden stress. Using the data for maximum particle velocity and empirical relationships developed from the Northridge earthquake, 1994 the distributions of the expected pipe breaks and red-tagged buildings for Sofia city are shown.

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The problem of site dependent estimation of the seismic strong motion for Sofia region is discussed. Deterministic modelling for micro-zonation of Sofia region is implemented for receiving an estimation of local site conditions as result of displacements provoked by seismic events in Sofia region with  stochastic and neural modelling. An approach for seismic waves classification on the base of principal axis transformation, long-range dependence time series analysis and neural modelling is suggested. With the help of scene oriented model are determined the boundaries of destructive phase of strong motion acceleration. For selected diapason of destructive phase is implemented two-dimensional vector quantization for transformed accelerograms with maximal and middle values. With self-organizing map are determined weight centres of selected classes of destructive phase. Learned and trained self-organizing map optimize selected target classes and  determine probability density function. Suggested model helps to provide side dependent estimation of the seismic strong motion for Sofia region.

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A deterministic analytical procedure for ground motion modelling, combining both modal summation and mode coupling techniques has been implemented to obtain synthetic seismic signals at Russe, NE Bulgaria, due to  one of the strongest Vrancea intermediate-depth earthquakes, which occurred during the last century, May  30, 1990. The frequency content of the synthetic signals in different frequency ranges, up to 1 and 2 Hz, has been studied separately. The results of this study, i.e. time histories and related ground motions parameters, can be used for different earthquake engineering analyses, e.g. structural performance assessments.

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The city of Sofia has the densest population of Bulgaria — around 1.27 million people. The capital is exposed to high seismic risk. According to the Bulgarian seismic code, Sofia has been included in a seismic category characterized by intensity IX (MSK), which corresponds to a horizontal acceleration of 0.27 g for the anchoring of the elastic response spectrum. Because of the lack of instrumental seismic data, realistic synthetic strong motion waveforms for two opposite positions of seismic sources, have been generated for an expected earthquake scenarios ( M = 7) along three geological profiles. A hybrid modeling method has been used for the modeling, which is based on the modal summation technique and finite difference scheme. The calculation has been done using an extended source model. The site effect is represented in terms of response spectra ratio (RSR), with respect to a bedrock model. The three components synthetic seismograms, computed in the domain of displacement, velocity and acceleration have been processed to extract some parameters very useful for engineering applications.

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This work focuses on the assessment of seismic risk and the need of long-term studies on the territory of the only terrestrial Bulgarian salt deposit (Provadia, NE Bulgaria, 27.43E, 43.2N) in connection with the observed higher seismic activity and probable manifestations of technogenic seismicity in the region.The necessity of regular monitoring of the geodynamic situation, surface subsidences and control of the mining excavations are discussed. A review of the manifestations of some observed problems due to the long-term mining is done. Some results of the performed monitoring at the site during the 1989–2009 are provided. Results of the analyses of the geological and seismotectonic situation, assessment of the stress-strain state of the salt diapir, surface subsidence, analysis of the recorded accelerograms, analyses of the seismic regime variation are discussed. The use of these analyses and their possible applications for the general stability assessment of the cavern-pillar system is formulated.

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This work focuses on the assessment of seismic risk issues associated with the potassium salt deposit of Provadia and Soligorsk, with the monitoring provided by Belarusian and Bulgarian specialists in the framework of a bilateral project.  The long-term studies 1983-2005 of the only terrestrial Bulgarian salt deposit (Provadia, φ = 43.06ºN, λ = 27.45ºE) and Belarusian (Starobin φ = 52.84ºN, λ = 27.47ºE) in connection with the observed higher seismic activity and probable manifestations of technogenic seismicity in the region is presented.  The characteristic features of the seismic processes as the identity of the curves of recurrence of seismic events of the energy range of 4-8 are discussed. A quasi-periodic character of the seismicity activation in time against the general trend of seismicity activation is established. It is shown that the zones of the epicenters of the seismic events are larger that mining areas.  Some differences in the pattern of seismic processes, such as: 1. the seismic activity in the range of small energies (K=4-8) is higher in the Soligorsk region; 2. events of the higher energy class, K≯9 are characteristic of the Provadia region. 

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Karst regions are very vulnerable with respect to natural and human-activity related hazards. Some of them with beautiful caves were proposed to be of national value. The examination of broken and slim intact speleothems in Bulgarian caves (Varteshkata and Elata caves, western part of Balkan Mountain Range, North-West Bulgaria) allows estimating an upper limit for horizontal peak ground acceleration (PGA) generated by paleo-earthquakes.The density, the Young’s modulus and the tensile failure stress of the samples originating from broken speleothems have been measured in a laboratory.The fundamental frequency and damping of speleothems have been measured in cavity by in situ observations.The value of the upper limit horizontal ground acceleration resulting in failure and the natural frequency of speleothems were assessed by theoretical calculations using mechanical parameters — the density, the Young’s modulus and the tensile failure stress — of the samples originating from a broken speleothem from Elata cave.The ages of the samples taken from the investigated stalagmites have been determined by alpha spectrometry.The pure elastic behaviour in analytical modeling and calculations have been used.According to our modeling results the investigated speleothem has not been excited by a horizontal acceleration higher than 0.144 g in case of Varteshkata cave, and 0.326 g in case of Elata cave during the last few thousand years. These results can serve to improve the present seismic risk policy for karst regions, too.

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