DBPapers
DOI: 10.5593/SGEM2016/B13/S05.084

THE NECESSITY OF CONSIDERING NONLINEAR SEISMOLOGY IN SITE EVALUATION

S.F. Balan, B.F. Apostol
Tuesday 6 September 2016 by Libadmin2016

References: 16th International Multidisciplinary Scientific GeoConference SGEM 2016, www.sgem.org, SGEM2016 Conference Proceedings, ISBN 978-619-7105-57-5 / ISSN 1314-2704, June 28 - July 6, 2016, Book1 Vol. 3, 663-670 pp

ABSTRACT
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.

Keywords: nonlinear, seismology, site evaluation, soil parameters, Vrancea region.