Importance of crustal structure and anelastic attenuation for estimating ground motion parameters by finite difference simulation

Abstract

We carry out simulations of seismic wave propagation in anelastic media in order to study the relative importance of source parameters as well as viscoelastic structures in affecting the decay of the ground motions. First, we verify the efficiency of the implementation in a finite difference code of two coarse-grain memory variables methods to model the anelastic behavior of the soil. We find that both methods are sufficiently consistent for a quality factor (Q) larger than 20. Then, we study the relative importance of the focal mechanism, the magnitude, the source depth, the crustal structure and the quality factor in affecting the decay of the ground motions. We verify that the magnitude and the focal mechanism of the source do not have a significant effect on the decay, whereas the focal depth is more important in explaining the variations in decay. The variations of the decay depending on the crustal structure are more difficult to assess. For the shorter distances (up to about 20 km), the velocity structure does not have a significant effect on the decay of the ground motions. The effect of the quality factor is perceptible but remains less important than the effect of the focal depth. However, for epicentral distances larger than about 20 km, both the velocity structure and the quality factor begin to affect significantly the decay. The effect of the quality factor on the decay becomes then even more important than the effect of the focal depth. Therefore, a reduction in the standard deviation of GMPEs could possibly be achieved through taking into account appropriate anelastic attenuation for each region considered. The effect of a 3D velocity structure is studied introducing a typical basin structure. The presence of a sedimentary basin can affect the decay even outside the basin. The spatial difference in ground motion is more pronounced in the elastic case than in the anelastic case.