Context

Recent devastating earthquakes — Turkey and Morocco in 2023 — have again underlined the need for reliable modelling and robust quantification of the impact of strong seismic motions on structures, and especially on critical infrastructure such as bridges, industrial facilities, storage tanks, nuclear energy buildings and tunnels.

Strong motions arise from the propagation of seismic waves through complex geological and geotechnical domains. They are intrinsically three-dimensional, and result from the combined effect of body waves (P, S) and surface waves (Rayleigh, Love, Stoneley). Yet state-of-the-art engineering practice for dynamic soil-structure interaction (SSI) still privileges a simplified method: seismic motions are assumed to arrive as body waves propagating vertically through idealised, horizontally-layered 1D soil columns — an approximation sometimes called “3×1D” (three orthogonal translational components obtained from a representative 1D soil column), as opposed to the real, full “3D” seismic motion.

This 1D approach is attractive because it is simple to solve and accurate enough for conventional civil engineering structures. Its main drawback is that it cannot model the generation and propagation of surface waves, which can amplify shaking and be particularly detrimental for structures of large dimensions (on the order of 100m) or with long periods of vibration. Real geological setups also rarely match the idealised infinite horizontal layering the 1D approach assumes, seismic waves show spatial variability and incoherence, and the underlying superposition assumption can fail under strong motions.

Building on ANR MODULATE

IM-SURF is a direct continuation of the recently completed ANR project MODULATE (grant 18-CE22-017), which investigated the effects of seismic motions with significant surface-wave components on several types of long-period structures — bridge piers, high-rise buildings and liquid storage tanks. Its main outcome was to show that the seismic response of such structures can be significantly amplified when surface waves are incorporated into the analysis — by up to a factor of about 3 for linear response, and by up to an order of magnitude for nonlinear response. MODULATE relied on a dedicated strong-motion database in which the surface-wave component of each signal could be identified and extracted, allowing full motions (body waves + surface waves) to be compared directly against motions without surface waves. It also shed light on how surface waves are generated in sedimentary basins, using realistic and canonical geometries and numerical techniques such as the Spectral Element Method (SEM) and the Boundary Element Method (BEM).

IM-SURF takes this work further: it extends MODULATE's strong-motion database, develops the numerical techniques needed to carry a full 3D wave regime from the geological scale down to a structure's foundation, and — for the first time in this line of work — applies the approach to massive structures of large dimensions with realistic foundation geometry, such as large industrial and nuclear buildings, rather than the simplified, long-period structural models used in MODULATE.