Vrancea Intermediate-depth Seismic Source Research Articles

Assessment of Liquefaction Hazard for Sites in Romania Using Empirical Models

This paper is focused on the evaluation of the liquefaction hazard for different sites in Romania. To this aim, a database of 139 ground motions recorded during Vrancea intermediate-depth earthquakes having moment magnitudes MW ≥ 6.0 is employed for the evaluation of the equivalent number of cycles for this seismic source. Several functional forms for the empirical evaluation of the equivalent number of cycles considering various seismological or engineering parameters are tested and evaluated. The regression analysis shows smaller uncertainties for the empirical models based on ground motion engineering parameters. Considering the lack of information in terms of engineering parameters, a simpler empirical model which accounts for the earthquake magnitude, source–site distance and soil conditions is selected for the liquefaction hazard analysis. Based on the proposed empirical model, specific magnitude scaling factors for Vrancea intermediate-depth earthquakes are proposed for the first time as well. The liquefaction hazard analysis is performed for sites whose seismic hazard is generated by either the Vrancea intermediate-depth seismic source or by local shallow crustal seismic sources. In the case of some of the selected sites, liquefaction phenomena were observed during past large-magnitude earthquakes. Unlike previous studies dealing with liquefaction analyses for sites in Romania, in this research, the hazard assessment is performed for various ground motion levels evaluated based on probabilistic seismic hazard assessment. Liquefaction hazard curves are constructed for each analyzed site. The results of the liquefaction hazard analysis show that this phenomenon is more likely to occur in the areas exposed to Vrancea intermediate-depth earthquakes, compared to the areas affected by local shallow earthquakes. In the case of the analyzed soil profiles from Bucharest, Craiova and Ianca, the minimum liquefaction safety factors less than one even for seismic hazard levels having mean return periods of 100 years and less.

A fast nonlinear dynamic analysis automated approach to produce fragility curves for 3D RC frames

This study describes an automated procedure to assess seismic fragility of 3D reinforced concrete frame structures. A number of 9600 3D nonlinear dynamic analyses are performed using the recently developed 3D Force Analogy Method, implemented in an automated Matlad-based non-commercial code. The numerical efficiency of 3D Force Analogy is first illustrated for a broad range of structures and groud motion recordings and scaling factors. The ground motion dataset consisting of 20 horizontal recordings used in the study is characteristic for soft deep soil sites (e.g Bucharest) which are affected by large magnitude earthquakes originating in the Vrancea intermediate-depth seismic source. A total of 24 RC frame structures, both regular and irregular are employed in the study. The Park-Ang damage index is employed in order to evaluate the damage states of the structures. Fragility functions corresponding to life-safety and collapse damage states are obtained for both principal directions of the structures. The results show similar median fragility values on both principal directions, and a larger variability on the transversal direction. The analyses have also shown that the standard deviations evaluated for the mean annual failure rates corresponding to the two considered damage states and on both principal directions is almost equal to the mean value. The mean value of the collapse probability evaluated for the peak ground acceleration with a mean return period of 2475 years and based on the results of all 24 models is 6.1 % for the longitudinal direction and 6.5 % for the transversal direction.

Seismic risk assessment of lifelines in Bucharest

This study is focused on the seismic risk assessment of lifelines from Bucharest, the capital city of Romania. The damage sustained by the lifelines in Bucharest during the major Vrancea intermediate-depth event of March 1977 is presented. The seismic exposure for the gas, electricity, sewage and water networks is evaluated using publicly available information collected from various sources. The seismic fragility is evaluated using the recommendations of the SYNER-G research project. Finally, seismic risk metrics, in terms of repair rates and probabilities associated with various damage states are evaluated using a Monte Carlo simulated earthquake catalogue for the Vrancea intermediate depth seismic source. The impact of considering the spatial correlation of ground motions and of the anchoring of the elements in the stations for the gas, electricity and water systems is also evaluated in this study. The largest mean probabilities associated with the moderate, extensive and complete damage states for Mw 7 and Mw 7.5 earthquake scenarios are obtained for the components of the electricity distribution system, namely electric substations and small generation plants. In addition, it has been also observed that the mean repair rate for the gas pipe network is smaller as compared to the water and sewage systems.

Recommended Path Durations for Stochastic Simulations of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Recommended Path Durations for Stochastic Simulations of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Evaluation of Key PSHA Assumptions—Case-Study for Romania

This case-study focuses on the analysis of several key assumptions necessary for the probabilistic seismic hazard assessment of Romania in the light of a future seismic hazard zonation of the territory. Among the aspects analyzed in this study are the appropriateness of the Poisson assumption which is tested on the earthquake catalogs of several seismic sources (crustal and the Vrancea intermediate-depth source), the azimuthal dependence of ground motion amplitudes from Vrancea intermediate-depth earthquakes and possible ground motion amplifications due to basin effects. The analyses performed in this study show that the Poisson distribution is able to model the observed earthquake frequency occurrence for the larger magnitude seismic events both for crustal and intermediate-depth seismic sources. Similar ground motion attenuation patterns irrespective of the azimuth with respect to the Vrancea intermediate-depth seismic source were observed only in the case of the 30 May 1990 earthquake, while in the case of the seismic events of 30 August 1986 and 31 May 1990 significant azimuthal ground-motion attenuation differences were observed. No significant differences in terms of ground motion amplitudes were observed at three seismic stations in Iasi area during the Vrancea intermediate-depth earthquakes of 30 May 1990 and 31 May 1990 possibly due to the limited elevation difference. Finally, significant long-period spectral amplifications were observed on the ground motions recorded at several sites from intramountainous depressions in Romania.

Seismic risk assessment of on-ground circular reinforced concrete and prestressed concrete water tanks using stochastic ground motion simulations

This case-study is dedicated to the seismic risk assessment of cylindrical fixed-based reinforced concrete and prestressed concrete water tanks situated in Romania. The fixed-based water tanks are designed according to the current seismic design prescriptions in Romania. The six selected sites are all under the influence of the Vrancea intermediate-depth seismic source. The seismic performance is evaluated using site-specific ground motion stochastic simulations and using the approach given in Eurocode 8-4 (2006). The largest mean annual failure probabilities are obtained for the two sites (Amara and Bucharest) with the highest long-period spectral amplifications. The study shows that from the strength point of view, the two types of tanks have very small corresponding failure probabilities and from the serviceability point of view, depending on the type of the tank and its position, sloshing or cracking due to bending are the prevalent failure modes.

Modeling energy release parameters in stochastic simulation of ground motions generated by Vrancea intermediate-depth seismic source

Ground motion time-domain stochastic simulations are performed by generating a white or a Gaussian noise, shaping it with a window, loading its normalized spectrum with the spectrum of the motion at a specific site and transforming it back in time-domain (Boore in Pure Appl Geophys 160: 635–676, 2003). The outcome of the resulted simulations is compared to the target parameters, namely amplitude, frequency content and duration parameters. Using SMSIM (Boore in SMSIM: Fortran programs for simulating ground motions form earthquakes, 2005) and EXSIM (Motazedian and Atkinson in Bull Seismol Soc Am 95(3): 995–1010, 2005. https://doi.org/10.1785/0120030207), Coțovanu (J Mil Technol 1(2): 27–32, 2018. https://doi.org/10.32754/jmt.2018.2.05) and Coțovanu and Vacareanu (J Seismol 24(1): 229–241, 2020. https://doi.org/10.1007/s10950-019-09892-5) applied the stochastic simulation method for generating ground motions produced by intermediate-depth Vrancea (Romania) earthquakes and found inconsistent results for duration parameters (significant duration and energy distribution in time-domain). In simulations, the duration of the motion results from the source and path duration, but the significant duration (that contains 90% of the energy) depends on the way the energy is distributed in time, and this distribution is controlled through a noise shaping window. The objective of this paper is to provide an appropriate form of the shaping window that can describe the specific energy release of ground motions generated by Vrancea intermediate depth seismic source (the first 50–70% of energy being rapidly released, the rest of it having a very slow release). For this matter, 371 recorded motions were analyzed and 5 types of energy release were defined. The set with the above described pattern that contained almost half of the analyzed accelerograms was used to define a mean normalized cumulative energy that describes the Vrancea specificity. On this basis, the parameters for the SMSIM implemented exponential window were determined and, because the results were inconsistent, a more adequate window function was defined and implemented in SMSIM. For the newly defined two-interval window function an algorithm for calculating the parameters was described, and the parameters of the mean release of energy were determined. By using the two-interval function for describing the Vrancea specific release of energy, more appropriate ground motion simulations resulted.

Continuity and Derivability Issues in Modeling the Energy Release Shaping Window in Stochastic Simulation of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Continuity and Derivability Issues in Modeling the Energy Release Shaping Window in Stochastic Simulation of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Local site conditions modeling in stochastic simulation of ground motions generated by Vrancea (Romania) intermediate-depth seismic source

Analyses of stochastic simulations methods for Vrancea (Romania) intermediate-depth earthquake recorded at seismic station INCERC Bucharest were made. The main research topic highlighted in this paper is the quest for the best model able to simulate the influence of the local site conditions considering to this aim different types of soil behavior. Two types of linear behavior and one type of nonlinear behavior of the soil layers were considered to investigate the site effects. Comparisons between the simulations and the recorded accelerograms and between the simulation methods were made in terms of amplitude parameters, frequency content, duration parameters, and energy release. A better approximation was found when the nonlinear behavior of the soil layers was employed.

Analysis of exceedance probabilities for design spectral accelerations from crustal earthquakes in Romania

The seismic hazard of the eastern and southern part of Romania is dominated by the Vrancea intermediate-depth seismic source, while for the western part of Romania the local crustal seismic sources are predominant. The design acceleration response spectrum of the Romanian seismic code is obtained by anchoring the design peak ground acceleration to a spectral shape which takes into account the soil conditions through the control period TC. In this study, we aim to evaluate the exceedance probabilities of the design spectral accelerations in the case of various low- and moderate-magnitude crustal earthquake scenarios. The evaluation is performed using an approach based on a ground motion prediction equation and an approach based on actual ground motion recordings. The results of the analyses show that significant differences in terms of exceedance probabilities of the design spectral ordinates occur between the two applied methodologies for larger magnitude earthquakes and small epicentral distances. Moreover, based on the results obtained, it appears that the minimum earthquake magnitudes causing exceedances of the design peak ground acceleration are quite far from the maximum magnitudes of the considered crustal seismic sources in the western part of Romania.

Seismic assessment for typical soft-storey reinforced concrete structures in Bucharest, Romania

In this study, a seismic assessment is performed for a low-code high-rise soft-storey reinforced concrete (RC) structure typical for Bucharest, Romania. One such RC structure collapsed in Bucharest during the March 1977 Vrancea intermediate-depth earthquake. Most of these structures were not retrofitted after the 1977 seismic event, they have been poorly maintained and have been affected by two additional Vrancea seismic events in 1986 and 1990. As such, considering that they are still inhabited by many Bucharest residents, the seismic assessment of these structures is an urgent matter. Consequently, a fragility analysis based on the results of typical pushover analyses, as well as based on incremental dynamic analyses is performed firstly. Moreover, a detailed sectional analysis regarding the seismic capacity of the vertical structural elements situated at the ground storey is also performed and highlights the brittle seismic behaviour of many RC vertical structural elements. The analyses also show that the structure has a larger strength in the transversal direction, but the displacement capacity is about half that for the longitudinal direction. Finally, the mean annual collapse probabilities computed using the ground motions from a Monte-Carlo simulated earthquake catalogue for the Vrancea intermediate-depth seismic source have the same order of magnitude on both principal directions of the structure, an observation confirmed by damage data collected after the 1977 Vrancea earthquake.

Stochastic Simulation of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Stochastic Simulation of Ground Motions Generated by Vrancea Intermediate-Depth Seismic Source

Collapse Assessment for Code-Conforming Reinforced Concrete Frame Structures in Romania

In this study, two RC frame structures having five stories and nine stories, respectively are designed using the current Romanian seismic code. The seismic performance is evaluated by integrating the site-specific seismic hazard with the structural fragility. The results show, as expected, a large scatter in the values of the collapse annual rates. The smallest annual collapse rates correspond to sites under the influence of shallow crustal seismic sources, while the largest ones are observed in sites under the influence of the Vrancea intermediate-depth seismic source. In addition, based on the results obtained, a larger seismic risk can be associated to the five-story structure. The study also shows that it is vital to develop a seismic hazard model that captures the long-period spectral amplifications observed at sites in southern Romania.

Life-Cycle and Seismic Fragility Assessment of Code-Conforming Reinforced Concrete and Steel Structures in Bucharest, Romania

In this article, the fragility of reinforced concrete and steel structures in Bucharest, a city of high seismic hazard, designed using the recent building codes in Romania, is assessed. A total of 24 reinforced concrete and steel structures with heights varying from five stories to 13 stories were analyzed. Their seismic fragility was evaluated using two procedures from the literature. In the first procedure (SPO2FRAG), the fragility was derived based on the pushover curves using the SPO2IDA algorithm, while in the second procedure (FRACAS), the fragility was derived from nonlinear time-history analyses. The analyzed structures were designed for three levels of peak ground accelerations, corresponding to mean return periods of 100, 225, and 475 years. Subsequently, the damage assessment of the analyzed structures was performed using ground motions generated from a Monte-Carlo simulated earthquake catalogue for the Vrancea intermediate-depth seismic source. The damage degrees that were estimated using the SPO2FRAG approach are higher than those estimated using the FRACAS approach. The life-cycle analysis of the structures shows that a further increase of the design peak ground acceleration for Bucharest is feasible from an economic point of view using the SPO2FRAG results. However, based on the FRACAS results, the opposite conclusion can be drawn. Finally, generic lognormal fragility functions are proposed as a function of building height and structural system.

Risk-targeted maps for Romania

Romania has one of the highest seismic hazard levels in Europe. The seismic hazard is due to a combination of local crustal seismic sources, situated mainly in the western part of the country and the Vrancea intermediate-depth seismic source, which can be found at the bend of the Carpathian Mountains. Recent seismic hazard studies have shown that there are consistent differences between the slopes of the seismic hazard curves for sites situated in the fore-arc and back-arc of the Carpathian Mountains. Consequently, in this study we extend this finding to the evaluation of the probability of collapse of buildings and finally to the development of uniform risk-targeted maps. The main advantage of uniform risk approach is that the target probability of collapse will be uniform throughout the country. Finally, the results obtained are discussed in the light of a recent study with the same focus performed at European level using the hazard data from SHARE project. The analyses performed in this study have pointed out to a dominant influence of the quantile of peak ground acceleration used for anchoring the fragility function. This parameter basically alters the shape of the risk-targeted maps shifting the areas which have higher collapse probabilities from eastern Romania to western Romania, as its exceedance probability increases. Consequently, a uniform procedure for deriving risk-targeted maps appears as more than necessary.

Assessment of seismic risk scenarios for Bucharest, Romania

In this paper, seismic risk scenarios for Bucharest, the capital city of Romania, are proposed and assessed. Bucharest has one of the highest seismic risk levels in Europe, and this is due to a combination of relatively high seismic hazard and a building stock built mainly before the devastating Vrancea 1977 earthquake. In this study, the seismic risk of Bucharest is assessed using the most recent information regarding the characteristics of the residential building stock. The ground motion amplitudes are evaluated starting from random fields obtained by coupling a ground motion model derived for the Vrancea intermediate-depth seismic source with a spatial correlation model. The seismic risk evaluation method applied in this study is based on the well-known macroseismic method. For several structural typologies, the vulnerability parameters are evaluated based on a damage survey performed on 18,000 buildings in Bucharest after the March 1977 earthquake. Subsequently, the risk metrics are compared with those from other studies in the literature that apply a different risk assessment methodology in order to gain a better view of the uncertainties associated with a seismic risk study at city level. Finally, the impact of several Vrancea intermediate-depth earthquake scenarios is evaluated and the results show that the earthquake which has the closest epicenter to Bucharest appears to be the most damaging.

Evaluation of the seismic hazard for 20 cities in Romania using Monte Carlo based simulations

This work focuses on the evaluation of the seismic hazard for Romania using earthquake catalogues generated by a Monte Carlo approach. The seismicity of Romania can be attributed to the Vrancea intermediate-depth seismic source and to 13 other crustal seismic sources. The recurrence times of large magnitude seismic events (both crustal and subcrustal), as well as the moment release rates are computed using simulated earthquake catalogues. The results show that the largest contribution to the overall moment release for the crustal seismic sources is from the seismic regions in Bulgaria, while the seismic regions in Romania contribute less than 5% of the overall moment release. In addition, the computations show that the moment release rate for the Vrancea subcrustal seismic source is about ten times larger than that of all the crustal seismic sources. Finally, the Monte Carlo approach is used to evaluate the seismic hazard for 20 cities in Romania with populations larger than 100,000 inhabitants. The results show some differences between the seismic hazard values obtained through Monte-Carlo simulation and those in the Romanian seismic design code P100-1/2013, notably for cities situated in the western part of Romania that are influenced by local crustal seismic sources.

Impact of spatial correlation of ground motions on seismic damage for residential buildings in Bucharest, Romania

This paper focuses on the investigation of seismic risk for residential buildings situated in Bucharest, the capital city of Romania. With a population of nearly 2 million inhabitants and a residential building stock of more than 130,000 units, Bucharest represents a city with one of the highest seismic risks in Europe. The seismic risk is evaluated taking into consideration only the Vrancea intermediate-depth seismic source which exhibits a dominant influence on the seismic hazard in southern and eastern Romania (including Bucharest, as well). The analyses are conducted based on a Monte Carlo earthquake catalogue which covers a time span of 50,000 years and includes more than 13,000 seismic events with magnitudes M W ≥ 6.0. In addition, in order to account for the ground-motion variability throughout Bucharest, a recently developed spatial correlation model is applied. Thus, the influence of the ground-motion correlation on the damage estimates for Bucharest is evaluated, as well. The seismic risk estimates obtained for various earthquake scenarios show a significant variability of the results obtained and offer valuable information for the decision-makers from public institutions dealing with the risk management.

Macroseismic intensity prediction equations for Vrancea intermediate-depth seismic source

This paper focuses on the development of a macroseismic intensity prediction equation (MIPE), for Vrancea intermediate-depth seismic source, that is able to differentiate between the sites located in the fore-arc and back-arc regions of the Carpathians Mountains. The database consists of more than 9000 observed data points (pairs of sites and macroseismic intensities) for six strong earthquakes originating from Vrancea intermediate-depth seismic source. Throughout this paper, the macroseismic intensities are expressed on MSK scale. The coefficients of the MIPE and the standard deviations of the model are obtained through the maximum likelihood method. The MIPE proposed for Vrancea intermediate-depth seismic source is valid for a magnitude range from M w 6.0 to M w 7.7 and for epicentral distances up to 1000 km in the fore-arc region and up to 800 km in the back-arc region. The inter- and intra-event residuals of the model are investigated, and magnitude and distance scaling are discussed. The predictive results obtained with the proposed MIPE are compared with other results from the literature. In the end, the correlation between the observed values of ground motion amplitudes and macroseismic intensities is discussed. In addition, an empirical relation between ground motion amplitudes and macroseismic intensities is obtained.

Assessment of the ground motion levels for the Vrancea (Romania) November 1940 earthquake

This short paper focuses on the evaluation of the ground motions levels likely experienced in ten cities seriously damaged during the November 1940 intermediate-depth Vrancea (Romania) earthquake (MW = 7.7). In the first step of the analysis, the ground motion levels are evaluated using a recently developed ground motion model (Vacareanu et al. in J Earthq Eng 19(3):535–562, 2015) for the Vrancea intermediate-depth seismic source. Next, the stochastic method of simulation (Boore in Pure App Geophys 160:635–676, 2003) is employed in the same purpose. In order to include the nonlinear soil effects into the results of the stochastic simulations, a dedicated amplification function is derived from the ground motions recorded during two large magnitude Vrancea earthquakes in 1986 and 1990. The results obtained for the November 1940 Vrancea earthquake show that the largest spectral ordinates are obtained when the nonlinear soil effects are taken into consideration, while the peak ground accelerations are in almost all cases in excess of 0.15 g. In addition, the results for Bucharest are validated through a site-specific response analysis performed using a soil profile from the literature. This site response analysis for the 1940 seismic event reveals long-period spectral amplitudes, similar to the ones observed in Bucharest during the Vrancea 1977 earthquake.