Soils exibit a strong non-linear behavior under cyclic loading conditions.This basic material characterisic shall be taken into account when we are making evaluating the seismic response of soil deposits or earth structures. Nonlinear effects in ground motion during large earthquakes have long been a contraversial issue between seismologists and engineers. Aki wrote in 1993: "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 conditions and this nonlinear
amplification" (Aki,A., Local Site Effects on Weak and Strong Ground
Motion, Tectonophysics, 218, pp.93-111, 1993). In other words, the seismological detection of the nonlinear site effects requires a simultaneous understanding of the effects of earthquake source, pro-pagation 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. In this XXI Century, "it will open up a new challenge for seismologists studiing nonlinear site effects in 2-D and 3-D irregular geological struc-tures, leading them to a fascinating research subject in nonlinear physics" (Aki, 1993, p.108, idem). The research component is in connection to assessment of the global and local seismic hazard (microzonation) through a linear and nonlinear seismology and
the vulnerability, resilience and finally, the real strength capacity of structures.

In detail the session focuses on the follwing aspects of nonlinear seismolgy:
1.Soil behavior during large earthquakes. Laboratory and field data.
2.The constitutive and dynamic equations. Approximated solutions of waves equations.
3.Quantitative evaluation of the nonlinearity from source to free field site.
4.Strong nonlinear dependence of spectral amplification factors of earthquake magnitude.
5.Assessment of earthquake hazard and risk in a real / nonlinear seismology.