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For assuring the stability of important buildings (nuclear power plants) during strong earthquakes, the soil strata under these buildings should not get deformed during the events. In the paper will be analyzed the nonlinear behavior of soil layers during strong earthquakes. As follows from the soil dynamics experiments, shear modulus G and damping D become functions of strain level γ and nonlinear behavior could take place. Further, changes could appear in the mechanical properties of the soil layers, expressed by their dynamic response during strong seismic movements. There will be presented researches and tests done at National Institute of R-D for Earth Physics. [ABSTRACT FROM AUTHOR]

The purpose of this research is to analyze the data recorded on instrumented structures in Bucharest and adjacent free-field seismic stations during the last moderate earthquake that occurred on 28th of October 2018 in the Vrancea seismic zone. The earthquake had a 5.5 moment-magnitude, a depth of 147.8 km, and it was felt on a large area of Romania, generating very little damage in Bucharest. In the last five years, the National Institute for Earth Physics has deployed permanent instrumentation (accelerometers) at key stories of five buildings in Bucharest metropolitan area that recorded the seismic motion of the mentioned earthquake. Three of the buildings have been equipped with seismic isolators and dampers and the performance of this earthquake-protection system was assessed based on earthquake data. The data recorded by four free-field seismic stations located in Bucharest were used as reference motion. In order to assess the amplification or reduction of the seismic waves when propagating in the structure, acceleration and velocity data were analyzed, both in time domain and in frequency domain, for selected frequency ranges. The paper presents also the acquisition setup, the dataflow and the data-processing methods. This study highlights the amplification / reduction of the seismic signal recorded at the ground level by the building structure. In the framework of previous studies on data recorded on buildings located in Bucharest, this study adds new valuable information and contribute to a better understanding of seismic response of buildings. [ABSTRACT FROM AUTHOR]

In this paper will be indicated the different behaviour of buildings in Bucharest, mistakes in design and execution, state of codes and regulations in the XXth century, revealing what went wrong. Several national and international research programs were done in the past to understand the causes for the Vrancea seismicity, to study the seismic wave propagation in the region and to assess seismic hazard for Bucharest and other communities. Especially in Bucharest, high-quality seismic data were acquired obtaining a unique dataset providing important information on the seismic amplitude variations across the area. For site studies of important constructions (social-cultural, administrative, industrial etc.) the designer must follow certain steps to ensure taking into account all the elements that could influence the reliability of future buildings. For this purpose a lot of investigations of seismological evaluations across the city have been carried out with new results given to construction developers. [ABSTRACT FROM AUTHOR]

The paper presents accelerations, response spectra and spectral ratio for two tower-type buildings, one in Southern part of Bucharest, T1, at ~160 km from the epicenter zone, and other, T2, near the epicenter zone, Vrancea. These seismic characteristics are all from recorded earthquakes triggered in the above mentioned area. 3 triaxial seismic accelerometers were used on 3 levels at each building. The accelerations recorded on the analyzed structures were a result of Vrancea medium magnitude earthquakes between 2014-2017 years, with magnitudes Mw ranging from 3.8 to 5.6, and focal depths between about 40 km to 148 km. An analysis of structural response was made concerning each building, T1 on 12 seismic events and T2 on 6 ones, and with 4 common seismic events for both structures. Maximum accelerations at three buildings levels and acceleration response spectra at the basement of buildings are calculated for two strongest earthquakes, and spectral ratio (top/base) for three, respectively two strongest seisms. The recorded data and subsequent analysis will contribute to a better understanding of the structures responses, even subjected to medium magnitude seismic events, and to the mitigation of seismic risk for densely populated areas. [ABSTRACT FROM AUTHOR]

Maximum damage due to strong earthquakes occurs at the resonance between the natural periods of the structure and its site, given by the coincidence (or just close to) of the two periods, in which case the dynamic amplifications can damage, even destroy, the structure. From the recordings of (strong) Vrancea earthquakes one can see a dependence of the site natural periods and peak ground accelerations (PGA) on magnitude. For safe avoidance of the resonance it is necessary to define for each site a natural period value range corresponding to strong earthquakes. In the range of MGR=7-7.5, which are considered as destructive magnitudes, with assigned PGA values of approximately ~0.1 to 0.3g and dangerous natural period range between 1.22 up to ~1.65s for INCERC site, the recommendation is to avoid buildings with these resonance characteristics. But for a large majority of sites only seismic recording of low and moderate events are available. The evaluation of the fundamental period for strong earthquakes using only seismic data implies an extrapolation procedure with inherent large errors. The resonant column determination by interpolation of the nonlinear variations in normalized form together with the determination of the normalization value T0 from seismic recording can lead to a better approximation of the natural periods for large PGA values. The paper shows ways of obtaining the site nonlinear natural period from in situ measurements, T0, and from resonant column data, Tn(PGA). With the help of normalized natural period functions Tn = Tn(PGA) we can predict natural period values of sites for strong earthquakes, starting from small and moderate seismic events, which gives us a good possibility in mitigation of seismic risk for future buildings. [ABSTRACT FROM AUTHOR]

The technique named "base isolation", resorts to place at the bottom of buildings special components, leading to long fundamental periods that would avoid resonance tendency of buildings with its site in the case of strong earthquakes. The nonlinear characteristics of the isolator layer change the linear period-shift estimation and can lead either to dangerous shortening shift in case of hardening nonlinearity or to unnecessary shift enlargement in case of softening nonlinearity. By neglecting this period-shift dispersion of the base isolation technology - the drawing out of structure from dangerous resonant zone - can be compromised. The vulnerability of a structure to strong earthquakes can be appreciated by comparing its fundamental period with the fundamental period of soil deposit. Under high stresses the rocks became nonlinear dependent of their dynamic characteristics. Oscillating systems with non-linear behavior (as a consequence of materials) have multiple resonances. This behavior imposes a new assessment for period-shift due to nonlinearity. The dispersion of the natural periods with respect to the loading is an important factor. In the same time, the period-shift from a fixed-base to an isolated base of the same structure depends on the nonlinear characteristics of the isolator and site layers. By neglecting the nonlinear aspects, a base-isolated structure may be brought in resonant conditions even if the structural and isolated layer natural periods are different. [ABSTRACT FROM AUTHOR]

Nonlinear seismology is that part in earth physics which considers nonlinear behavior in soil response during strong earthquakes, our case study. Nonlinearity in soil response is also called strain dependence and in geotechnical engineering, soil response is nonlinear over a certain level of deformation. In field observations, in order to show nonlinear effects, the spectral amplification factor (SAF) has been calculated for Vrancea earthquakes, by a group of researchers from our institute (including the present authors), as the ratio between the maximum values of the spectral absolute acceleration (Sa), relative velocity (Sv) and relative displacement (Sd) for a fraction of critical damping at fundamental period to the corresponding peak values (acceleration, velocity and displacement) obtained from the processed strong motion records. As the main evidence of this nonlinearity phenomenon for the Romanian case of strong intermediate-depth earthquakes (from Vrancea region) it was observed a decrease in the variability of peak ground acceleration for increasing earthquake magnitudes. This phenomenon is due to the propagation of seismic waves through large sedimentary basins, as the case where the Romanian capital, Bucharest, is placed. Site examples will be presented. Soil tests from our laboratory of soil mechanics from National Institute for Earth Physics made with resonant columns Drnevich and Hardin show the nonlinear behavior of soils as a consequence of the decrease of shear modulus (G) and increase in damping ratio (D) with increasing strain γ (in real world this is caused by strong earthquakes) at strain values above 10-4%. All these features will be analyzed in the paper with theoretical and practical assessments (examples of tests on soil samples) showing the necessity and benefits of considering nonlinear seismology in site evaluation for the mitigation of seismic risk during strong earthquakes. [ABSTRACT FROM AUTHOR]

During the ongoing project "Spaceborne Multiple Aperture Interferometry and Sequential Patterns Extraction Techniques for Accurate Directional Ground and Infrastructure Stability Measurements" (DGI-SAR), between : "National Institute of RD for Earth Physics" (NIRDEP) and "Terrasigna Ltd" we learned that we can deliver to local authorities, in our case the Municipality of Bucharest and Nuclear Electric Plant at Cernavoda, Romania, important information about the safety of existing buildings through satellite monitoring. These data will be delivered through the service named: "Critical Infrastructure and Seismic Movement Monitoring Service", whose organization will be presented in the paper. Also we shall present other aspects concerning the safety of buildings during their lifetime. On a map of Bucharest, advanced InSAR interferometric techniques help locate certain regional or local anomalies exemplified by ground uplifting or subsidence. On such a map we overlap seismic intensities, periods of oscillation of Bucharest zones during a strong earthquake which could affect the behavior of constructions of the studied zone/region. This showed us possible endangered areas during an earthquake. Persistent Scatterer Interferometry (PSI) technique gave us the possibility to find out about settelemnts of some buildings and sites. This is an important factor for integrity of buildings during their lifetime and especially when they are in seismic regions, because settlement could occur because of soft soil or other reasons in the lifetime of a construction. Moreover, for the first time in the SAR industry, with the launch of the Sentinel-1 Constellation in 2014 continuous spatial and temporal (every 12 days) coverage of the whole Earth is provided and the data is available directly and free of charge. This is a great opportunity to define a robust service based on this mission. First Sentinel-1 InSAR results for Bucharest City will be presented in this paper. The satellite monitoring of areas of interest, metropolitan and industrial, gave us data, which processed, lead to mitigation of seismic risk and discover soil movements, like settlements, during the lifetime of a construction which could endanger its safety. [ABSTRACT FROM AUTHOR]

This work focuses on using SAR (Synthetic Aperture Radar) satellite data in order to help mitigate the seismic risk of Bucharest Metropolis, the capital of Romania. The city has more than 2.5 million inhabitants and it is considered after Istanbul the second-most earthquake-endangered metropolis in Europe. This study is part of an ongoing joint project between National Institute of R-D for Earth Physics (NIRDEP) and Terrasigna Ltd. NIRDEP is taking care of the seismic network and data processing for the whole Romania, having a consistent number of seismic digital stations also in Bucharest. Terrrasigna applied the PSI technique to extract the ground deformation in Bucharest during July 2011 - January 2013, from a long time-series of SLC TerraSAR-X images. Based on the ground deformation satellite map, we can identify some seismic risk (hazard and vulnerability) in Bucharest Metropolis. The map reveals local ups and downs on the city surface, which could have different causes. If there is a larger settlement and structural damage begins, even almost unnoticed and an earthquake of magnitude more than 7 would come, (3 of such events were in Romania in the XX-th century: in the year 1940 with magnitude M w = 7.6; year 1977, with magnitude Mw = 7.4, with almost 1400 people dead only in Bucharest, year 1986, with magnitude Mw = 7.1,) damage of the building could occur. That is why it is very useful to know, maybe with years in advance that a large settlement occurred and the buildings there could be affected by a strong earthquake. [ABSTRACT FROM AUTHOR]