GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.2 GNGTS 2023 Innovative application of the Multi-Lapse Time Window Analysis (MLTWA) to vertical array data M. Salilih 1,2 , S. Parolai 1,2 , V. Poggi 2 1 University of Trieste, Department of Mathematics and Earth sciences 2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale- OGS Abstract Seismic wave attenuation is an intrinsic property of Earth materials, causing energy dissipation as seismic waves propagate through earth materials. Seismic scattering and intrinsic seismic energy attenuation contribute to the observed wave attenuation. The separation of the two effects is necessary for improving ground motion simulations necessary for seismic hazard and risk studies. Several methods have been proposed to determine the relative amounts of scattering loss and intrinsic absorption. Among them, the Multi-Lapse Time Window Analysis (MLTWA) allows for estimating the relative contribution of scattering loss and intrinsic absorption to total S-wave attenuation from S-wave envelopes. According to Fehler (1992), MLTWA is based on two assumptions: a) that the early portion of the seismogram is dominated by the direct S-wave, whose amplitude is controlled by the total attenuation of the media. b) that S-coda is composed entirely of scattered S-waves whose amplitudes are controlled by the total scattering coefficients According to Aki (1969} seismic coda results from the scattering of seismic waves by random heterogeneity in the Earth's lithosphere. Several different scattering theories have been developed for modelling seismic coda envelope. Wu (1985) was the first to take multiple scattering into account to measure the relative contributions of scattering and intrinsic absorption to total attenuation. Hoshiba et al. (1991) gave the numerical basis of the method by computing theoretical S-wave energy density from a Monte Carlo simulation of multiple isotropic scattering. Then Hoshiba (1997) modified the method to numerically (using a Monte Carlo approach) compute the theoretical energy envelopes for a stratified earth model. The MLTWA, that is implementing the Hoshiba (1997 ) approach, is based on the comparison of the seismic energy integrated in three consecutive time windows and displayed against hypo-central distance, with the integrals predicted by a theoretical model suitable for the multiple-scattering problem. Therefore, the strategy of the MLTWA method is to simultaneously minimize the discrepancy between the computed energy integrals and those predicted by theory.