Characteristics of Surface Ground Motion in Beirut, Lebanon, from 3D Seismic Wave Simulation

Site effects arising from complex two- and three-dimensional (2D/3D) site geometries and highly heterogeneous soils cannot be fully captured by traditional one-dimensional (1D) ground response analyses. At geotechnical and sedimentary scales, these effects have been shown to significantly influence earthquake ground motion, leading to amplification or deamplification, duration lengthening, and spatial variability of surface ground motion. In this study, we investigate how complex surface geology and topography affect ground motion characteristics in Beirut, Lebanon. We build a detailed 3D velocity model of the city and its suburbs, incorporating geological, geophysical, and topographical data to reveal large variations in shear-wave velocity structure and seismic bedrock depth. We then conduct 3D numerical simulations of seismic wave propagation up to 5 Hz using spectral-element methods assuming horizontally polarized SH plane waves. We validate our 3D velocity model by comparing simulated and observed site amplifications. Using several ground motion indicators, we identify significant spatial variation of surface ground motion as well as large site amplification and associated ground motion duration lengthening caused by 2D/3D site effects throughout the city. Our findings, based on synthetic ground motions, provide a robust basis for improving seismic damage assessment in Beirut in the event of future earthquakes.