Vrancea Intermediate-depth Earthquakes Research Articles

Influence of Rotating Strong Ground Motions on the Response of Doubly Symmetrical RC Wall Structures in Romania and Its Implication on Code Provisions

This research focuses on the evaluation of the seismic response of three doubly symmetrical RC wall structures having three height regimes and subjected to a series of bi-directional ground motion recordings. The recorded strong ground motions are rotated in a clockwise direction and the effects of this procedure on several engineering response measures (e.g., wall-bending moments, wall shear forces, and storey drifts) are evaluated. The three doubly symmetrical RC wall structures are designed according to the current Romanian seismic design code for a peak ground acceleration of 0.25 g. The strong ground motions used in this study consist of five pairs of recordings from the Vrancea intermediate-depth earthquakes of 1977, 1986, and 1990 and from the Erzincan and Aigion earthquakes in Turkey and Greece. The investigations show differences between the response measures as a function of the orientation of the strong ground motion horizontal components and significant differences between the responses of each individual structure, as well. Thus, the maximum response obtained from the rotation procedure can be regarded as dependent on both the ground motion characteristics and on the ground motion recording, as well.

Investigation on regional attenuation of Vrancea (Romania) intermediate-depth earthquakes

This paper aims at investigating possible regional attenuation patterns in the case of Vrancea (Romania) intermediate-depth earthquakes. Almost 500 pairs of horizontal components recorded during 13 intermediate-depth Vrancea earthquakes are employed in order to evaluate the regional attenuation patterns. The recordings are grouped according to the azimuth with regard to the Vrancea seismic source and subsequently, Q models are computed for each azimuthal zone assuming similar geometrical spreading. Moreover, the local soil amplification which was disregarded in a previous analysis performed for Vrancea intermediate-depth earthquakes is now clearly evaluated. The results show minor differences between the four regions situated in front of the Carpathian Mountains and considerable differences in attenuation of seismic waves between the forearc and backarc regions (with regard to the Carpathian Mountains). Consequently, an average Q model of the type Q(f) = 115×f 1.25 is obtained for the four forearc regions, while a separate Q model of the type Q(f) = 70×f0.90 is computed for the backarc region. These results highlight the need to evaluate the seismic hazard of Romania by using ground motion models which take into account the different attenuation between the forearc/backarc regions.

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.

Broadband Ground Motion Simulation of the 2004 and 1977 Vrancea, Romania, Earthquakes Using Empirical Green’s Function Method

We estimated the parameters of strong motion generation areas and simulated broadband ground motions for the moderate October 27, 2004 (M w 5.8) and damaging March 4, 1977 (M w 7.4) Vrancea (Romania) intermediate-depth subduction earthquakes using the empirical Green’s function method. The method allows the simulation of ground motions in a broadband frequency range by summing up the subevent records, corresponding to small magnitude events in the near-source areas, which are assumed to follow the source-scaling relationship and the omega-square source spectral model. We first estimated the strong motion generation area that reproduces near-source ground motions in a broadband frequency range of 0.3–10 Hz for the 2004 earthquake, by fitting the synthetic acceleration, velocity, and displacement waveforms to the observed data. The source properties of the obtained strong motion generation area are in agreement with the predictions made using an empirical source scaling relationship for crustal earthquakes, implying a stress drop of approximately 10 MPa for the 2004 earthquake. We then modeled the strong motion generation area for the 1977 damaging earthquake using the 2004 earthquake as an empirical Green’s function and constructing a source model based on its estimated source parameters. To simulate the unique record at Bucharest, capital city of Romania, the rupture was assumed to have propagated from the northeast bottom of the strong motion generation area having a stress drop of 50 MPa. Broadband ground motion simulations were further compared in terms of the modified Mercalli intensity values, calculated from the peak ground accelerations and peak ground velocities of synthetic waveforms, with the observed Medvedev–Sponheuer–Karnik intensity values. Our estimates of the source properties for the 2004 and 1977 Vrancea intermediate-depth earthquakes support the size-dependent stress drop.

Investigation on the Variability of Simulated and Observed Ground Motions for Bucharest Area

ABSTRACTIn this research, we evaluate the variability of ground motions from Vrancea intermediate-depth earthquakes for Bucharest area. In the first step of the analysis, the variability of ground motions recorded during four intermediate-depth Vrancea seismic events is assessed. Subsequently, the variability of ground motions simulated using the stochastic method is evaluated. The soil conditions in Bucharest are taken into consideration using the horizontal-to-vertical spectral ratios computed from the ground motions recorded during Vrancea seismic events with magnitudes MW ≤ 6.0. The stochastic simulations are performed for earthquakes with magnitudes in the range MW = 5.5–7.5 and four focal depths (90, 110, 130, and 150 km), while the epicentral positions take into account the instrumental seismicity of the 20th century. The results show similar mean normalized acceleration response spectra for the 13 analyzed sites in Bucharest area, as well as a significant influence of the earthquake magnitude on the spectral shapes. A comparison of the simulated ground motions with the amplitudes obtained from a ground motion model derived for the Vrancea subcrustal seismic source shows much larger spectral amplitudes for the former category. In addition, the largest part of the ground motion variability can be attributed to earthquake magnitude.

Characteristics And Results Of The Near Real-Time System For Estimating The Seismic Damage In Romania

Abstract The Near Real-Time System for Estimating the Seismic Damage in Romania, implemented in 2012 at the National Institute for Earth Physics, is one of the automated systems that can directly contribute to saving many lives right after a major earthquake, by translating earthquake parameters into damage probabilities for different areas within Romanian counties and showing emergency intervention necessities, and can also lead to mitigation actions before an earthquake, through raising awareness and highlighting vulnerable aspects of the building stock and economic and social impacts. This paper aims to present the scientific background of this constantly upgrading system, and to show different results for relevant scenarios, for intermediate-depth Vrancea earthquakes and other crustal earthquakes. Several important questions are tried to be answered, like: “How credible are the estimated losses?”, “What are the most vulnerable aspects?” or “How can the damage maps be useful for authorities?”. Currently, the system uses for building loss estimation the analytical methods (as the Improved-Displacement Capacity Method - I-DCM) implemented within the open-source software SELENA (SEismic Loss EstimatioN using a logic tree Approach), together with HAZUS methods for estimating the human casualties. The building stock is defined through 48 different capacity and fragility curves, depending on construction material, height and age. As hazard data, PGA and SA values obtained through the ShakeMap System and based on real recordings and attenuation relations are used. The area currently analyzed by the system consists of 19 Romanian Counties, capital Bucharest and 9 regions in northern Bulgaria; resolution of the data is at administrative unit (commune or city) level. We aim to provide an insight of each part of this system, justify the choices made and also discuss the improvement possibilities.

Source Effects on the Spectral Characteristics of Strong Ground Motions Recorded in Bucharest Area During Vrancea Earthquakes of 1986 and 1990

The August 30, 1986 (MW = 7.1) and May 30, 1990 (MW = 6.9) Vrancea intermediate-depth earthquakes, despite their almost similar magnitudes, have produced very different spectral contents as shown by the strong ground motions recorded in Bucharest and its surroundings. The differences can be attributed to different epicentral distances and to different values of the stress drop. The characteristics of the seismic ground motions recorded in Bucharest area in the 1986 and 1990 seismic events are discussed in the context of (a) the source characteristics of the two earthquakes and (b) the local soil conditions in Bucharest. Furthermore, an attempt is made to determine the soil factors S defined in EN 1998-1 for the Bucharest area, based on the strong ground motion dataset recorded during the two seismic events.

Slowness and azimuth determination for Bucovina array (BURAR) applying multiple signal techniques

The BURAR seismic array, located in Northern Romania (Bucovina region), is designed to monitor events located in an area poorly covered by other existing seismic stations. In order to use the BURAR array for single-station locations, it is crucial to calibrate the azimuth and slowness parameters, which are currently used in array techniques to locate earthquakes, blasts or nuclear explosions. The goal of this study is to apply “f–k” and plane wave fit techniques in order to constrain the slowness and azimuth parameters at BURAR for teleseismic, regional and local events. The analysis was carried out using P and S waves recorded for events occurred between 2004 and 2008 within a radius of 50° around BURAR. The azimuth values obtained applying both methods strongly deviated from the theoretical values for regions like Central Turkey, Bulgaria, Dodecanese Islands and other parts of Greece, while the ray parameter deviations with respect to a 1-D IASP91 reference model are less significant. For the local events, the anomalies are smaller, except the particular case of Vrancea intermediate-depth earthquakes for which strong azimuth deviations (33.5°), both positive and negative, are observed. We investigate how these systematic deviations in azimuth are explained by the structure lateral heterogeneities which characterize the study region.

Advanced real-time acquisition of the Vrancea earthquake early warning system

The Vrancea seismogenic zone in Romania represents a peculiar source of seismic hazard, which is a major concern in Europe, especially to neighboring regions of Bulgaria, Serbia and Republic of Moldavia. Earthquakes in the Carpathian–Pannonian region are confined to the crust, except the Vrancea zone, where earthquakes with focal depth down to 200 km occur. One of the cities most affected by earthquakes in Europe is Bucharest. Situated at 140–170 km distance from Vrancea epicenter zone, Bucharest encountered many damages due to high energy Vrancea intermediate-depth earthquakes; the March 4, 1977 event ( M w=7.2) produced the collapse of 36 buildings with 8–12 levels, while more than 150 old buildings were seriously damaged. A dedicated set of applications and a method to rapidly estimate magnitude in 4–5 s from detection of P wave in the epicenter were developed. They were tested on all recorded data. The magnitude error for 77.9% of total considered events is in the interval [−0.3, +0.3] magnitude units. This is acceptable taking into account that the magnitude is computed from only 3 stations in a 5 s time interval (1 s delay is caused by data packing). The ability to rapidly estimate the earthquake magnitude combined with powerful real-time software, as parts of an early warning system, allows us to send earthquake warning to Bucharest in real time, in about 5 s after detection in the epicenter. This allows 20–27 s warning time to automatically issue preventive actions at the warned facility.

Estimates of some ground motion parameters, related to the damage potential of strong intermediate-depth Vrancea earthquakes

This study is related to the particular strong intermediate-depth Vrancea earthquakes. Its purpose is to confirm once again the significant influence of the seismic source mechanism to the seismic input at a given site through estimates of different engineering parameters, used to describe the damage potential of the ground motion due to an earthquake. Real and synthetic time-series have been used in the computations. The synthetic seismic wavelets have been computed applying the neo-deterministic seismic hazard assessment procedure and the relevant software and hardware facilities at the Department of Earth Sciences, University of Trieste, Italy. To derive the ground motion parameters of interest, all seismic signals have been uniformly processed by the Seismosignal software.

Periodic upward migration model for intermediate-depth earthquakes in Vrancea, Romania

Vrancea intermediate-depth earthquakes with a depth range of 60–160 km occur frequently. By using data from ROMPLUS catalog, we analyze earthquakes with M ≥ 7.0 that occurred during 1500–2000. We propose a periodic upward migration (PUM) model for these earthquakes. According to this model, (1) active zones at the bottom of seismic regions migrate upward with the velocity V, with the activity starting from the year T S and, (2) this migration repeats every T P years, and at a given depth, seismic active period continues for T1 years. We test the model using the likelihood function and compare the model with a uniform Poisson model. A simulation using the bootstrap method indicates that the PUM model for M ≥ 7. 5 and M ≥ 7. 3 earthquakes is statistically significant at the 5% level and is better than the Poisson model. We propose the following regularity for Vrancea M 7 intermediate-depth earthquakes. Three earthquakes occur at a depth of 140–160 km, 110–140 km, and 80–110 km at the beginning, at the middle, and at the end of each century, respectively. This phenomenon repeats every century. The next M 7 earthquake is expected to occur at a depth of 140–160 km soon.

Shake Map Methodology for Intermediate-Depth Vrancea (Romania) Earthquakes

We establish and test a shake map methodology for intermediate-depth Vrancea earthquakes, based on seismological information gathered in Romania during recent years. We use region- (azimuth-) dependent attenuation relations derived from stochastic simulations of ground motions using spectral models of Vrancea earthquakes. Both region boundaries and Fourier amplification spectra for the characterization of seismic site effects are based on several hundred weak, moderate and strong-motion records and macroseismic intensity maps. We determine region-specific, magnitude- and distance-dependent amplification factors of peak values and instrumental intensity relative to rock. We interpolate recorded ground motions and ground motion estimates from the obtained amplification factors and attenuation relations for rock conditions. The resulting shake maps show a good agreement with macroseismic descriptions of moderate-sized and large Vrancea earthquakes, demonstrating the feasibility of a seismological approach to shake map generation. Unlike previous methodologies, this approach requires neither expensive assessments of geology-dependent site amplification factors, nor large numbers of strong-motion records. Our results are in good agreement with empirical topographic slope-site amplification relations, but give a better reflection of the abnormal attenuation of seismic waves in the Transylvanian region and the strong amplification in the Focsani basin.

Source Spectra and Site Response from S Waves of Intermediate-Depth Vrancea, Romania, Earthquakes

Abstract Seismograms from 55 intermediate-depth Vrancea earthquakes ( M 4.0–7.1) recorded at 43 stations of an accelerometric network in Romania are used to derive source spectra and site amplification functions from S waves in the frequency range 0.5–20 Hz with the generalized inversion technique (GIT) (Castro et al. , 1990). Attenuation is taken into account using the nonparametric attenuation functions derived by Oth et al. (2008) from the same dataset, and the attenuation-corrected data are then split into source and site contributions. The source spectra follow the ω -2 model (Brune, 1970, 1971) with high corner frequencies and a related Brune stress drop of the order of 100 MPa. The site amplification functions are separately determined for both horizontal and vertical components. Contrary to widespread expectation, the vertical component shows significant amplification effects at high frequencies. The H / Z ratios determined from the GIT results compare well with H / V ratios computed directly from the S -wave window of the accelerograms (Lermo and Chavez-Garcia, 1993). The basic assumption for the determination of site effects from H / V ratios is that the vertical component is not or only little affected by site effects. For Vrancea earthquakes, this assumption is incorrect; consequently, site effects should not be estimated from H / V ratios. The reason for this peculiar fact is the geometry of intermediate-depth seismicity that leads to almost vertical ray paths beneath the stations.

Probabilistic seismic hazard in terms of intensities for Bulgaria and Romania – updated hazard maps

Abstract. Since 2007 Bulgaria and Romania are members of the European Union. All member states have to introduce the European earthquake building code EUROCODE 8 (EC 8) in the coming years. Therefore, new seismic hazard maps have to be calculated according to the recommendations in EC 8. Here the authors present a novel approach to compute such hazard maps. We prefer to use the macroseismic intensity as hazard parameter because of two reasons: - The irregular azimuthal attenuation pattern of the Vrancea intermediate depth earthquakes can be advantageously taken into account by using detailed macroseismic maps. - The intensity is directly related to the degree of damage and is the original information in the historical earthquake catalogues. The main base of our probabilistic analysis is the earthquake catalogue for SE-Europe (Shebalin et al., 1998) in combination with national and regional catalogues. Fore- and aftershocks were removed. Seismic source zones inside an area of about 200 km around Romania and Bulgaria were defined based on seismicity, neotectonics and geological development. For each seismic source the intensity-frequency relation was calculated and a maximum possible earthquake as well as a seismogenic depth was estimated. An appropriate attenuation law was assumed. To cope with the irregular isoseismals of the Vrancea intermediate depth earthquakes, a factor Ω was included in the macroseismic attenuation law. Using detailed macroseismic maps of three strong intermediate depth earthquakes, Ω was calculated for each observation. Strong local variations of Ω are avoided by averaging within grid cells of 0.5 degree in longitude and 0.25 degree in latitude. The contributions of all seismic sources, the crustal normal depth source zones and the Vrancea intermediate depth zone, were summed up and the annual probability of exceedance was calculated. The contribution of the Vrancea intermediate depth zone to each grid point was computed with the corresponding representative Ω of this point; a seismogenic depth of 120 km has been assumed. Each final seismic hazard map is a combination of two maps, the one for normal depth source zones and the one for the Vrancea intermediate depth zone. This is illustrated for a recurrence period of 475 years. Additional hazard maps were calculated for different recurrence periods.

Inversion of the October 27, 2004 Vrancea (Romania) earthquake using teleseismic waveforms data

Several source parameters (depth, scalar moment, rupture velocity, focal mechanism) are determined for the moderate-size October 27, 2004 (MW = 5.8) Vrancea (Romania) intermediate-depth earthquake by inversion of teleseismic waveforms.

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

We present the regional ground-motion prediction equations for peak ground acceleration (PGA), peak ground velocity (PGV), pseudo-spectral acceleration (PSA), and seismic intensity (MSK scale) for the Vrancea intermediate depth earthquakes (SE-Carpathians) and territory of Romania. The prediction equations were constructed using the stochastic technique on the basis of the regional Fourier amplitude spectrum (FAS) source scaling and attenuation models and the generalised site amplification functions. Values of considered ground motion parameters are given as the functions of earthquake magnitude, depth and epicentral distance. The developed ground-motion models were tested and calibrated using the available data from the large Vrancea earthquakes. We suggest to use the presented equations for the rapid estimation of seismic effect after strong earthquakes (Shakemap generation) and seismic hazard assessment, both deterministic and probabilistic approaches.

Dynamic properties of the Quaternary sedimentary rocks and their influence on seismic site effects. Case study in Bucharest City, Romania

Bucharest is one of the cities most affected by earthquakes in Europe. Situated at 150–170 km distance from Vrancea epicentral zone, Bucharest had suffered many damages due to high energy Vrancea intermediate-depth earthquakes. For example, the 4 March 1977 event produced the collapse of 32 buildings with 8–12 levels, while more than 150 old buildings with 6–9 levels were seriously damaged. The studies done after this earthquake had shown the importance of the surface geological structure upon ground motion parameters. New seismic measurements are performed in Bucharest area aiming at defining better elastic and dynamic properties of the shallow sedimentary rocks. Down-hole seismic measurements were performed in a number of 10 cased boreholes drilled in the Bucharest City area. Processing and interpretation of the data lead to the conclusion that shallow sedimentary rocks can be considered weak in the area, down to 150–200 m depth. Seismic wave velocity values and bulk density values presented in the paper associated with local geology are useful primary data in the seismic microzonation of Bucharest City. They are used as 1D models to derive transfer functions and response spectra for the stack of sedimentary rocks in several parts of Bucharest area, leading to a better knowledge of the local site amplification and associated frequency spectra. In a recent study the H/V spectral ratio using Nakamura’ s method was applied on the seismic noise measurements in 22 sites in Bucharest City in order to derive the fundamental period associated with these sites. The values confirm the previous results, showing a dominant resonance in the period range of 1.25–1.75 s. The fundamental periods obtained with Nakamura’ s method are in good agreement with those computed on the basis of geological and geotechnical data in boreholes, which show an increase of the fundamental period in the Bucharest area from south to north, in the same direction as the increase of the thickness of the Quaternary deposits above the Fratesti layer which is considered the bedrock in the area.

On the recurrence time of earthquakes: insight from Vrancea (Romania) intermediate-depth events

SUMMARY The statistics of earthquake inter-occurrence times in broad areas have been reported to obey universal scaling laws (e.g. by Bak et al. and Corral). We investigate the recurrence times of intermediate-depth Vrancea earthquakes (depth >60km), taking advantage of the scarcity of aftershocks for deeper events. We use a complete earthquake catalogue (M ≥ 2.8) spanning from 1974 to 2002. The analysis of the probability density distribution of the recurrence times for the entire catalogue reveals a power-law regime at small scales. In order better to understand this scaling, we also analyse declustered catalogues, from which the distinct aftershock sequences following the 1977, 1986 and 1990 major earthquakes have been removed, and a time period without large events. For these data sets, the recurrence times follow an exponential distribution at all scales. This observation suggests that deviations from the exponential model ‐ characteristic of the scaling laws recently proposed ‐ result primarily from short-term aftershock clustering.

Source parameters of intermediate-depth Vrancea (Romania) earthquakes from empirical Green's functions modeling

Several source parameters (source dimensions, slip, particle velocity, static and dynamic stress drop) are determined for the moderate-size October 27th, 2004 ( M W = 5.8), and the large August 30th, 1986 ( M W = 7.1) and March 4th, 1977 ( M W = 7.4) Vrancea (Romania) intermediate-depth earthquakes. For this purpose, the empirical Green's functions method of Irikura [e.g. Irikura, K. (1983). Semi-Empirical Estimation of Strong Ground Motions during Large Earthquakes. Bull. Dis. Prev. Res. Inst., Kyoto Univ., 33, Part 2, No. 298, 63–104., Irikura, K. (1986). Prediction of strong acceleration motions using empirical Green's function, in Proceedings of the 7th Japan earthquake engineering symposium, 151–156., Irikura, K. (1999). Techniques for the simulation of strong ground motion and deterministic seismic hazard analysis, in Proceedings of the advanced study course seismotectonic and microzonation techniques in earthquake engineering: integrated training in earthquake risk reduction practices, Kefallinia, 453–554.] is used to generate synthetic time series from recordings of smaller events (with 4 ≤ M W ≤ 5) in order to estimate several parameters characterizing the so-called strong motion generation area, which is defined as an extended area with homogeneous slip and rise time and, for crustal earthquakes, corresponds to an asperity of about 100 bar stress release [Miyake, H., T. Iwata and K. Irikura (2003). Source characterization for broadband ground-motion simulation: Kinematic heterogeneous source model and strong motion generation area. Bull. Seism. Soc. Am., 93, 2531–2545.] The parameters are obtained by acceleration envelope and displacement waveform inversion for the 2004 and 1986 events and MSK intensity pattern inversion for the 1977 event using a genetic algorithm. The strong motion recordings of the analyzed Vrancea earthquakes as well as the MSK intensity pattern of the 1977 earthquake can be well reproduced using relatively small strong motion generation areas, which corresponds to small asperities with high stress drops (300–1200 bar) and high particle velocities (3–5 m/s). These results imply a very efficient high-frequency radiation, which has to be taken into account for strong ground motion prediction, and indicate that the intermediate-depth Vrancea earthquakes are inherently different from crustal events.

Probabilistic seismic hazard map for Bulgaria as a basis for a new building code

Abstract. A seismic hazard map proposed as part of a new building code for Bulgaria is presented here on basis of the recommendations in EUROCODE 8. Seismic source zones within an area of about 200 km around Bulgaria were constructed considering seismicity, neotectonic and geological development. The most time consuming work was to establish a homogeneous earthquake catalogue out of different catalogues. The probabilistic seismic hazard assessment in terms of intensities is performed following Cornell (1968) with the program EQRISK (see McGuire, 1976), modified by us for use of intensities. To cope with the irregular isoseismals of the Vrancea intermediate depth earthquakes a special attenuation factor is introduced (Ardeleanu et al., 2005), using detailed macroseismic maps of three major earthquakes. The final seismic hazard is the combination of both contributions, of zones with crustal earthquakes and of the Vrancea intermediate depth earthquakes zone. Calculations are done for recurrence periods of 95, 475 and 10 000 years.