THE QUANTITATIVE EVIDENCE OF THE SOIL NONLINEAR BEHAVIOR DURING OF STRONG VRANCEA EARTHQUAKES IN REAL / NONLINEAR SEISMOLOGY

Nonlinear effects in ground motion during large earthquakes have long been a controversial issue between seismologists and geotechnical engineers. Laboratory tests made by using resonant columns consistently show the reduction in shear modulus (G) and increase in damping ratio (D) with increasing shear strain (γ), i.e., G = G(γ), respectively, D = D(γ), therefore nonlinear viscoelastic constitutive laws are required.Aki (1) wrote: "Nonlinear amplification at sediments sites appears to be more pervasive than seismologists used to think…Any attempt at seismic zonation must take into account the local site condition and this nonlinear amplification". In other words, the seismological detection of the nonlinear site effects requires a simultaneous understanding of the effects of earthquake source, propagation path and local geological site conditions. The difficulty for seismologists in demonstrating the nonlinear site effects has been due to the effect being overshadowed by the overall patterns of shock generation and propagation. The authors, in order to make evidence of nonlinear effects, introduced the spectral amplification factor (SAF) as ratio between maximum spectral absolute acceleration (Sa), relative velocity (Sv), relative displacement (Sd) and peak values of acceleration (amax), velocity (vmax) and displacement (dmax), respectively, from processed strong motion record. The evidence for nonlinearity at least for thick Romanian Plain Quaternary sediments is a systematic decrease in the variability of peak ground acceleration with the increasing earthquake magnitude.