GNGTS 2015 - Atti del 34° Convegno Nazionale

1 to 10 Hz. P- and S- input polarizations were kept separately; first I ran a modeling with a S- source input polarization, then I repeated the computation using a P- source polarization. The output synthetics computed along the surface contain both the polarization (P- and S- waves) due to the interferences between input pulse and waves diffracted from the irregular geometries. I used the internal mesher of specfem2d respecting the time and spatial discretization rules, obtaining 270000 reference rectangular elements and 4324801 grid points for each single model mesh. The left, right and bottom sides of each model were absorbing boundaries using on the whole about 6000 PML (Perfectly Matched Layers) spectral elements, with mean thickness of the absorbing layer around 30 m. Accordingly, the spurious waves generated from the boundaries show negligible energy. The compressional (Vp) and shear-wave velocity (Vs) of the models were fixed to 2000 and 1000 m/s, respectively. The quality factors Q for P- and S- waves were assumed to be 200 (damping of 0.25%) and 100 (damping of 0.5%), following the usual (velocity/10) rule of thumb. The density was fixed to 2 g/cm 3 and the Poisson’s ratio was 0.33. The computational dimension of each model was 9000·2000 m (x and z direction, respectively). The receivers were in number of 250 and were situated along the uppermost surface, with a spacing between adjacent receivers of about 12 m. The snapshots at different time steps (Fig. 1) and the stack of the signal traces show clearly the effect of irregular geometries on the seismic waves. The half-circle, the slope and the triangle distort the planar input, acting as diffractors of energy and generating waves diffracted from the geometrical irregularities propagating along the surface. On the left- and right- part from the basic geometries, the waves travel symmetrically along the flat surface. The symmetry of Fig. 1 – Basic geometries used in the 2-D P-SV modeling: half-circle, slope and triangle. A planar up-going and down- going Ricker wave is propagating into the model ( specfem2d tool). Red and blue colors indicate the negative and positive pulse of seismic wave, respectively. The plot refers to S source polarization, and the snapshots correspond to a time step of 1.5 sec after the beginning of the simulation. 88 GNGTS 2015 S essione 2.2