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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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The investigations presented in this paper were aimed at empirical definition  of ground motion under Vrancea earthquakes.  They were based on the recorded accelerogrammes from the occurred strong Vrancea earthquakes of 1977, 1986 and 1990 obtained from the accelerographs installed in the territory of former Yugoslavia, Romania and Bulgaria. A methodological approach to empirical prediction of ground motion parameters under strong earthquake effects was developed and empirical attenuation laws of horizontal peak ground acceleration (PGA) were defined.  A new empirical mathematical model was applied. In this model the amplitudes of strong ground motion are in function of earthquake magnitude, epicentral distance, focal depth, azimuth of the instrument location in respect to  the radiation pattern and the ratio between the semi-axes of the seismic field  ellipse. Through the so called non-homogeneity function of the region, the model indirectly involves the effect of the focal mechanism and the non-homogeneity of the region through which the seismic waves propagate.  The mathematical model applied in these investigations contributed to the  empirical definition of the attenuation laws that play an important role in seismic  hazard analyses and hence in evaluation of the seismic hazard a country or a  region is exposed to.  The results obtained from these investigations are important not only for   Macedonia but for the entire Balkan region and beyond.  The presented methodology and the applied mathematical model of functional relationships are of a particular importance since they are different from the empirical models of strong ground motion that have so far been applied in the world.

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Assessment of seismic safety of nuclear power plants requires convolution of plant fragilities with seismic hazard curves. Plant level fragility versus peak ground acceleration is obtained by combining component fragilities according to the Boolean-expression of the sequence leading to core damage. An improvement of the fragility modeling can be achieved via description of the fragility as a function of cumulative absolute velocity of ground motion instead of peak ground acceleration. In the paper the physical meaning and dependence on strong motion parameters is also discussed. The reason why the cumulative absolute velocity is an appropriate damage indicator is analyzed.

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A newly developed technique is presented to estimate site specific seismic failure frequencies of several types of storage tanks. Seismic failure is a failure event triggered by an earthquake leading to material release and serious damages. The present method is based on the convolution of site specific seismic hazard curves and component dependent fragility curves. Fragility curves are calculated with the help of probit coefficients originated from the analysis of historical data of earthquake damage. The calculation considers only the filling level, pressure and the damage state of storage tanks, and neglects the shape, volume, material and other parameters. Determination of seismic failure frequencies for storage tanks is done for three different Hungarian plant sites (Paks, Püspökszilágy, Csömör) using seismic hazard curves. In contrast of the simplifications the obtained frequency values represent a conservative estimation of failure frequencies triggered by earthquakes, and in comparison to international failure data the calculated values indicate significant frequencies. Therefore, the results imply that despite of the moderate seismic activity in Hungary, the effects of earthquakes on major-accident hazards should be considered in quantitative risk assessment.

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We applied deterministic and probabilistic methodologies for seismic hazard assessment at Tirana city, expressed in terms of horizontal peak ground acceleration (PGA). The deterministic evalues of PGA are determined according to the scenario R = 6.4 km and M max = 6.0, while the probabilistic ones have been assessed applying the smoothed seismicity approach. The results that derive from the two methodologies show equal values of PGA for the selected site inside Tirana city and higher than the PGA values established by the official maps as well as the actual seismic coefficients that are being applied by the actually in force national design code KTP-N.2–89 for Tirana area.

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The source parameters and dimensions of the tow strongest earthquakes (July 1, 1829, I o = VII-VIII EMS; October 15, 1834,  I o = IX EMS) in Érmellék area are estimated as follows  Date of the event     Focal depth     Magnitude    Rupture area          Max.                                                                                                       displacement July  01, 1829           21-33 km         5.5-5.7         33-55 km2        11-16 cm October 15, 1834     23-28 km         6.5-6.6        266-358 km2      74-90 cm  The average recurrence that we may expect an earthquake of M ≥ 0.7 every 1 year, an earthquake of M ≥ 2.9 every 10 years and an earthquake of M ≥ 5.0 every 100 years in this source zone. The probabilistic seismic hazard assessment predicts 1.1-1.4 m/cm2 peak ground accelerations, and 6.3-7.4 maximum (theoretical) earthquake intensity values with 10 % chance of exceedance for an exposure time of 100 years in the area.

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Ground motion models represent empirical mathematical-physical models of ground motion under strong earthquakes defined on the basis of databanks of past earthquake records. Within these, the ground motion parameters as dependant random variables are related to the earthquake parameters, the seismic energy transfer characteristics of the region through which the seismic waves propagate and the influence of the local soil conditions as independent random variables. Regression analysis, either single or double, can be applied as a method for their determination. They are used in deterministic and probabilistic methods for prediction and estimation of the expected seismic action of future earthquakes upon the investigated location or region.Ground motion models are very important for prediction of future seismic actions and have been paid special attention. Following the analysis of a variety of published ground motion models for shallow and subduction earthquakes carried out by different authors, there are predictive equations, methods, synthesis, discussions on the state of the art and new trends in investigations of ground motion models.In this study, the author’s attention has been focused on the new ground motion model dependant on the azimuth, which the author has developed and applied for determination of peak ground acceleration attenuation laws for the Vrancea intermediate earthquakes (Stamatovska 1996, 2002, 2006, Stamatovska and Petrovski 1997). Besides the already published results for the peak ground acceleration attenuation laws-PGA for the locations Bucharest, Cherna Voda, and Valeni, the author presents, for the first time, the progress of the applied ground motion model dependent on azimuth in determination of the response spectrum attenuation laws for the Vrancea intermediate earthquakes for Cherna Voda and Bucharest locations.

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In the seismic source zone of Bánát more than 600 earthquakes are known since 1773 among them six events with magnitude of 5.0–5.7 measured on the surface magnitude scale. The macroseismic reinterpretation of the April 2, 1901 earthquake yields epicentral intensity of VII on the European Macroseismic Scale, and a focal depth value of 12 km. Based on empirical relations the maximum rupture area is estimated as 50–55 km 2 and the maximum displacement along the fault is about 16 cm in the Bánát seismic zone due to the M S = 5.7 event occurred on July 12, 1991. The average recurrence that we may expect an earthquake of M ≥ 3.4 every 1 year, an earthquake of M ≥ 4.3 every 10 years and an earthquake of M ≥ 5.3 every 100 years in the studied source zone. The probabilistic seismic hazard assessment predicts 1.3–2.1 m/sec 2 peak ground accelerations, and 6.7–7.3 maximum (theoretical) earthquake intensity values with 10% chance of exceedance for an exposure time of 100 years in the region.

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The source parameters and dimensions of the three most important earthquakes (Komárom 1763, Io = IX EMS; Komárom 1783, Io = VII-VIII EMS; Mór 1810, Io = VIII EMS) of the area studied are estimated as follows     event                        focal depth   rupture area    max. displacement Jun. 28, 1763 Komárom   7.6 km     93 - 125 km2   29 - 35 cm Apr. 22, 1783 Komárom  11.5 km     18 -  36 km2   6 - 11 cm Jan. 14, 1810 Mór             5.0 km     18 -  45 km2   6 - 14 cm The average recurrence that we may expect an earthquake of M=2.7 every 1 year, an earthquake of M=4.0 every 10 years and an earthquake of M=5.3 every 100 years in this source zone. The probabilistic seismic hazard assessment predicts 1.4-2.0 m/cm2 peak ground accelerations, and 6.9-7.2 maximum (theoretical) earthquake intensity values with 10% chance of exceedance for an exposure time of 100 years in the area.

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The source parameters and dimensions of the three most important earthquakes (Zsolna - Jan. 15, 1858, I o = VIII EMS; Jóko - Jan. 9, 1906, I o = VIII EMS; Jóko- Jan. 16, 1906, I o = VII-VIII EMS) of the area studied are estimated as follows: Event (Magnitude, Rupture area, Max. Displacement); Jan. 15, 1858 Zsolna (5.5 Ms, 22-36 km2, 8-11 cm); Jan. 9, 1906 Jóko (5.7 MS, 40-55 km2, 12-16 cm); Jan. 16, 1906 Jóko (5.3 MS, 12-24 km2,4-8 cm). The average recurrence that we may expect an earthquake of M = 2.3 every 1 year, an earthquake of M = 3.7 every 10 years and an earthquake of M = 5.1 every 100 years in this source zone. The probabilistic seismic hazard assessment predicts 1.2-1.7 m/sec2 peak ground accelerations, and 6.6-7.2 maximum (theoretical) earthquake intensity values with 10% chance of surpassing for an exposure time of 100 years in the area.

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