Abstrak

Differences between 3-D numerical predictions of earthquake ground motion in the Myg donian basin near Thessaloniki, Greece, ledus to deﬁne four canonical stringent models derived from the complex realistic 3-D model of the Myg donian basin. Sediments atopanelastic bedrockare modelled in the 1D-shar pand1D-smooth models using three homogeneous layers and smooth velocity distribution, respectively. The 2D-sharp and 2D-smooth models are extensions of the 1-D models to anasymmetric sedimentary valley. In all cases, 3-D wave ﬁelds include strongly dispersive surface waves in the sediments. We compared simulations by the Fourier pseudospectral method (FPSM), the Legendre spectral-element method (SEM) and two formulations of the ﬁnite-difference method (FDM-S and FDM-C) up to 4Hz. The accuracy of individual solutions and level of agreement between solutions vary with type of seismic waves and depend on the smoothness of the velocity model. The level of accuracy is high for the body waves in all solutions. However, it strongly depends on the discrete representation of the material interfaces (at which material parameters change discontinuously) for the surface waves in the sharp models. An improper discrete representation of the interfaces can cause inaccurate numerical modelling of surface waves. For all the numerical methods considered, except SEM with mesh of elements following the interfaces, a proper implementation of interfaces requires deﬁnition of an effective medium consistent with the interface boundary conditions. An orthorhombic effective medium is shown to signiﬁcantly improve accuracy and preserve the computational efﬁciency of modelling. The conclusions drawn from the analysis of the results of the canonical cases greatly help to explain differences between numerical predictions of ground motion in realistic models of the Mygdonian basin. Were commend that any numerical method and code that is intended for numerical prediction of earthquake ground motion should be veriﬁed through stringent models that would make it possible to test the most important aspects of accuracy.