On the effects of an unknown depth-to-bedrock in seismic site response analyses for deep-water sites

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Seismic Site Response Analysis (SSRA) plays a key role in defining design earthquake motions at the ground surface or at the structure foundation level. Standard practice consists in (1) performing a Probabilistic Seismic Hazard Analysis (PSHA) to define the ground motion at the seismic bedrock, and (2) quantifying the effects of shallow sediments on the ground motion by means of a SSRA. Although such an approach is well rooted in engineering practice, it requires the knowledge of the seismic bedrock characteristics as well as the sediment cover properties, which represent a challenge for deepwaters sites. Unlike onshore sites, where one can often rely on a detailed subsurface profile, offshore sites often face limitations due to depth-limited investigations, leading to difficulties in accurately modelling the seismic wave propagation. Whilst stochastic analyses can partially overcome these issues by introducing modelling uncertainties in the input parameters, the need of time-domain analyses to capture the soil nonlinear behaviour for large shear strains (>1%) may prevent the use of such methods for offshore sites.

Motivated by the challenges experienced in industrial projects, the main aim of this paper is to contribute to the development of best practices to be applied when performing SSRA for offshore structures. By performing 1D total-stress non-linear SSRA for a real case-study, the paper seeks to understand the influence of bedrock modelling on SSRA results for deepwaters sites in earthquake-prone areas.

5th International Symposium on Frontiers in Offshore Geotechnics (ISFOG2025)

1 - Site investigation strategies, geohazards and integrated studies