Ⓔ Near-Fault Earthquake Ground-Motion Simulation in the Grenoble Valley by a High-Performance Spectral Element Code by Marco Stupazzini, Roberto Paolucci, and Heiner Igel Abstract 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 applica- tions. Coupling near-fault conditions with site effects induced by complex geological structures (such as deep alluvial basins or steep topographic irregularities) further con- tributes 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 non- linear range, was used for this purpose; full advantage was taken of its implementation on parallel computer architectures. After introducing GeoELSE and its parallel per- formance, and after introducing some of its validation benchmarks, the spatial varia- bility 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 con- ditions 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. Online Material: Movie of simulated wavefield and hypocenter locations. Introduction Near-fault earthquake ground motion displays specific features in terms of amplitude, duration, and frequency con- tent (Somerville et al., 1997; Rodriguez-Marek and Bray, 2006) that directly affect the seismic response of structures, especially in the nonlinear range (see, e.g., Hall et al., 1995; Chopra and Chintanapakdee, 2001; Mavroeidis et al., 2004) and may have a significant impact in the overall distribution of damage. This was made clear by numerous observations from recent earthquakes such as Northridge, California (1994), Kobe, Japan (1995), Kocaeli, Turkey (1999), Chi- Chi, Taiwan (1999), and Bam, Iran (2003). Although the amount of available near-fault records has increased recently, thanks to the Japanese K-Net (available at http://www.k‑net.bosai.go.jp/) and the 2004 M w 6 Parkfield, California, earthquake (Shakal et al., 2006), the near-fault earthquake ground-motion dependence on the focal mecha- nism, the rupture propagation characteristics, and the com- plex interaction with the Earth’ s shallow structure is still hard to predict. Furthermore, accounting for the previous effects from the numerical point of view implies large-scale 3D simula- tions of seismic-wave propagation, including the combined effects of a properly characterized seismic source, a reason- ably detailed shallow crustal model, and the near-surface complex geological or topographical irregularities. Such nu- merical simulations are computationally very heavy; thus, relatively few research groups are able to develop the capa- bilities to perform them. A list of selected research works, 286 Bulletin of the Seismological Society of America, Vol. 99, No. 1, pp. 286–301, February 2009, doi: 10.1785/0120080274