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 one-dimensional (1D) soil models that are 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 article, we present a parametric study of the seismic response of the Grenoble Valley, France (due to an M-w 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; full advantage was taken of its implementation on parallel computer architectures. After introducing GeoELSE and its parallel performance, and after introducing some of its validation benchmarks, the spatial variability of the seismic response of the Grenoble Valley is quantitatively investigated taking into account two effects: (i) the hypocenter location and (ii) the nonlinear soil behavior through a nonlinear viscoelastic 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 unfavorable interaction between fault rupture, radiation mechanism, and complex geological conditions may give rise to large values of peak ground velocity (exceeding 1 m/sec) even in low-to-moderate seismicity areas; it may therefore considerably increase the level of seismic risk, especially in highly populated and industrially active regions, such as the Alpine valleys.