Path effects and local elastic site amplification: two case studies on Mt Etna (Italy) and Vega Baja (SE Spain)

Local site effects, normally ground motion amplification, represent one of the main components when developing ground motion simulations and play an important role in the potential earthquake damage. In the framework of the UPStrat-MAFA project a stochastic finite-fault simulation method was selected for the generation of synthetic ground motion scenarios. This method uses spectral site correction functions to account for site amplification effects. These local effects may undergo significant changes due to the source–receiver configuration (i.e., distance, source depth and ray incidence). This holds in particular for reflection and transmission coefficients which may strongly vary depending on the source–receiver geometry, and may alter the characteristics of the spectral site-correction functions. A strategy is proposed to account for local site effects in the context of the regional geological structure, considering SH-waves propagating in a 1D velocity model. Spectral correction functions are derived by comparing Green’s functions obtained for general velocity models and those more detailed at shallow depths. The developed approach is applied in two of the test areas selected in the project, the Mt Etna in Italy and the Vega Baja in SE Spain. The results show the different behaviour in two environments, i.e., volcanic and tectonic, with different seismicity characteristics, and highlight the importance of performing specific site-effect studies in some regions where standard building code soil factors could have some limitations to evaluate the potential for ground motion amplification.