Near-fault earthquake ground motion simulation in the Grenoble Valley by a high-performance spectral element code

Near-fault effects are known to produce specific features of earthquake ground motion, such as long period velocity pulses and directivity, that cannot be predicted by numerical approaches involving vertical plane wave propagation in 1D soil models, used as a standard in engineering applications. Coupling near-fault conditions with site effects induced by complex geological structures, such as deep alluvial basins or steep topographic irregularities, further contributes to the complexity of earthquake ground motion and to the difficulty to provide reliable predictions without making use of large-size 3D numerical simulations. In this paper we present a parametric study of the seismic response of the Grenoble Valley, France, due to a MW 6 seismic source at some 10 km epicentral distance from the urban area, that was carried out in the framework of an international benchmark for earthquake ground motion prediction. The spectral element code GeoELSE for seismic wave propagation analyses in 3D heterogeneous media, in the linear and nonlinear range, was used for this purpose and full advantage was taken of its implementation on parallel computer architectures. After introducing GeoELSE and its parallel performance, and introducing some of its validation benchmarks, the spatial variability of the seismic response of Grenoble valley is quantitatively investigated taking into account two effects: (i) the hypocenter location and (ii) the nonlinear soil behaviour, through a nonlinear visco-elastic soil model. Finally, numerical results are compared with available data and attenuation relationships of peak values of ground motion in the near-fault region. Based on the results of this work, the unfavourable interaction between fault rupture, radiation mechanism and complex geological conditions may give rise to large values of peak ground velocity (exceeding 1 m/s) even in low-to-moderate seismicity areas, and therefore increase considerably the level of seismic risk, especially in highly populated and industrially active regions, such as the Alpine valleys.