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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]

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]