M. MISICU, GHEORGHE MARMUREANU, CARMEN O. CIOFLAN, BOGDAN F. APOSTOL
National Institute for Earth Physics, P.O. Box MG-2,077125, Bucharest, Romania
In order to determine the seismic effects in superficial layers of the earth taking into account their nonlinear constitutive behavior, specific wave equations for longitudinal and transversal propagations are deduced. Following approximations schemes previously enounced by the first author, which furnish effective solutions corresponding to a layered half-space as well as correlations between surface effects (displacements and stresses) and the dislocational mechanism of sources located in ground layer, in the present paper the amplification effect for a layered half-space with nonlinear behavior of the surface layer is analyzed. The results are in good agreement with observational data and consist in specific new effects: the existence of very well defined directivity curves and of almost punctual regions with focusing effects, their dependence on focal mechanism, deep rocks structure and wave superposition, the importance of constitutive laws, the nonlinearity of soft surface soils and of multiple wave overlapped simultaneously with increasing fundamental frequency. These facts are pointing out the correlation between nonlinearity and directivity and are encouraging the efforts for a new approach in viscous-plastic and fissional rocks behavior.
Key words: amplification effects, nonlinear behavior