Impact of MW definition approach on Fourier ground-motion variability of shallow crustal earthquakes in Europe

A Ground-Motion Model (GMM)'s apparent aleatory variability is inflated by errors in its pre-dictor parameters, specifically the moment magnitude ( MW ). Multiple MW values can be available for an event (direct or deduced) and various MW defini-tion approaches have been proposed to assign a unique MW value to an event. In this study, we investigate the impact of MW definition on a pan-European Engineering Strong Motion dataset based Fourier GMM, using two datasets with MW defined by two distinct approaches: [1] the ranking strategy of the Euro-Mediterranean Earthquake Catalogue (EMEC 2019) and [2] the multi-strategy (standardization, ranking, unification, averaging) approach to MW definition of Laurendeau et al., (Geophys J Int 230:1980-2002, 2022). Large discrepan-cies in MW values can be observed especially between MW ranging from 4.0 to 5.0. While the GMM median predictions remain unchanged irrespective of dataset, we report a large reduction in between-event variability of the GMM at low frequencies (< 1.8 Hz) when strategy [2] is adopted over [1] (18% at 0.35 Hz). This reduction applies to frequencies before the corner-frequency of the Fourier spectrum, as this part of the spectrum depends primarily on seismic moment. We attribute this reduction to the use of direct MW values in [2] instead of deduced MW values in [1], the priority scheme in the ranking strategy, and the unification strategy. Our study suggests that the approach used to define a unique MWin the GMM dataset may have a significant impact on its predictions in seismic hazard assessment. Highlights center dot Moment magnitude ( MW ) definition approach significantly influences the prediction variability of ground-motion models. A unification-ranked approach to defining MW reduces the pan-European variability by up to 18% at low fre-quencies. Centroid Moment Tensor services for MW are preferable to specific studies for statistic seismic hazard assessment.