Ground motion modelling with a stochastically perturbed excitation

We present a new hybrid method combining deterministic and stochastic featu res. The aim is to describe the crustal propagation better than determinist ic or stochastic methods can do separately. We start from the deterministic hybrid method based on Discrete-Wavenumber and Finite-Difference technique s (DW-FD). First we modify the DW-FD procedure by introducing topographical variations and a spatially varying Q factor. Then, to take into account ef fects due to small-scale heterogeneities of the crust, we add a stochastic noise (perturbation) to the deterministic signal propagated through the cru st. The stochastic noise is constructed using a kind of Markov-like process generator with two physical constraints: to have the Brune spectrum, and t o reproduce the spatial decay of coherence reported in literature for real sites. We have chosen a Markov-like technique because it allows us to get s tochastic noise, with the given coherence spatial decay, directly in time d omain. This new hybrid method is applied in a numerical test, the parameter s of which approximate the case of the 12 June 1995 Rome earthquake. It is found that the coherence decay with distance at the alluvial valley surface is slower than the prescribed coherence decay inside the bedrock.