NON-LINEAR SOIL RESPONSE AT STRONG MOTION OBSERVATION SITES WITH A FOCUS ON BOREHOLE ARRAY

behaviour in seismological data. It focuses mainly on vertical arrays as an exceptional naturallaboratory to study the propagation of seismic waves through the subsurface soil layers. The reviewhighlights the breakthroughs that were performed such as the demonstration of the significance of thenon-linear soil behaviour or the proposal of methodologies to back calculate the non-linear soilproperties.The PGA is one of the most used parameters to describe the level of seismic solicitations. The reportedPGA threshold from which non-linear soil behaviour is observed, has been decreasing since the 80'sthanks to the increase of available data, the improvement of the data quality, of the computationalefficiency. The results of all studies lead to (1) new methodologies to invert data and provide in-situcharacterisation of soil behaviour and (2) the definition of proxy parameters that reflect the impact ofnon-linear soil behaviour on site responses. From these proxies it is now possible to take into accountnon-linear soil behaviour in the analysis of site responses without using numerical simulations. Futureperspectives in this field concern the densification of the instrumented sites and the characterisation ofsoil properties to provide natural laboratories that are well-constrained. For the assessment of the insitunon-linear soil properties, the quantification of the uncertainties and spatial variability alsorepresent a challenge. To do so we need to gather even more the data which have been made availablethrough international collaborations.