3D numerical simulation and ground motion prediction: Verification, validation and beyond – Lessons from the E2VP project

Abstract The Euroseistest Verification and Validation Project (E2VP) is part of a series of complementary benchmarking exercises launched to better assess the ability of numerical simulation to accurately predict seismic ground motion. E2VP targeted more specifically the current, most-advanced numerical methods applied to realistic 3D, linear models of sedimentary basins through a quantitative comparison of the recorded and numerically-simulated ground motions. The target site, located within the Mygdonian basin near Thessaloniki, Greece, has been thoroughly investigated for two decades and a detailed, realistic 3D model has been derived from geological, geophysical and geotechnical investigations, while a dedicated instrumentation provided a significant number of surface and borehole recordings. Verification and validation tests up to a frequency of 4 Hz, much beyond the 0.4 Hz fundamental frequency of the deepest part of the graben, have been performed for a set of 19 local, small to moderate magnitude events. For careful and accurate enough computations, the model-to-model differences are smaller than the model-to-observations differences, the latter being controlled by uncertainties primarily in the crustal propagation model and source properties, and secondarily in the shallow structure. It is therefore recommended to prefer distant and/or deep events (R>10–20 km, Z>8–10 km) for validation exercises. Additional sensitivity tests illustrate the ability of carefully verified numerical simulation tools to provide an instructive insight at the structure of the so-called “aleatory” variability of ground motion, for both its within- and between-event components. The between-event variability is shown to be very sensitive to hypocenter location errors (even as low as ±2 km), and to uncertainty in magnitude estimates. It explains the increase of aleatory variability for small magnitude events and emphasizes the usefulness of dense seismological networks. The within event, single-site variability is shown to be associated to an “epistemic” dependence of the 3D site response on the event back-azimuth, distance and depth, and calls for caution when interpreting single-station variabilities derived from a too small number of events.