Quantitative Comparison of Four Numerical Predictions of 3D Ground Motion in the Grenoble Valley, France

Abstract

International audienceThis article documents a comparative exercise for numerical simulation of ground motion, addressing the seismic response of the Grenoble site, a typical Alpine valley with complex 3D geometry and large velocity contrasts. Predictions up to 2 Hz were asked for four different structure wave-field configurations (point source and extended source, with and without surface topography). This effort is part of a larger exercise organized for the third international symposium on the effects of surface geology (ESG 2006), the complete results of which are reported elsewhere (Tsuno et al., 2009). While initial, blind computations significantly differed from one another, a remarkable fit was obtained after correcting for some nonmethodological errors for four 3D methods: the arbitrary high-order derivative discontinuous Galerkin method (ADER-DGM), the velocity-stress finite-difference scheme on an arbitrary discontinuous staggered grid (FDM), and two implementations of the spectral-element method (SEM1 and SEM2). Their basic formulation is briefly recalled, and their implementation for the Grenoble Valley and the corresponding requirements in terms of computer resources are detailed. Besides a visual inspection of PGV maps, more refined, quantitative comparisons based on time-frequency analysis greatly help in understanding the origin of differences, with a special emphasis on phase misfit. The match is found excellent below 1 Hz, and gradually deteriorates for increasing frequency, reflecting differences in meshing strategy, numerical dispersion, and implementation of damping properties. While the numerical prediction of ground motion cannot yet be considered a mature, push-button approach, the good agreement reached by four participants indicates that, when used properly, numerical simulation is actually able to handle correctly wave radiation from extended sources in complex 3D media. The main recommendation to obtain reliable numerical predictions of earthquake ground motion is to use at least two different but comparably accurate methods, for instance the present formulations and implementations of the FDM, SEM, and ADER-DGM

Spectral-Element Simulations of Seismic Waves Generated by the 2009 L'Aquila Earthquake

“…Modeling lateral variations in wavespeed, density, and attenuation is of paramount importance in order to accurately reproduce ground motions, as documented in many studies (e.g., Olsen et al, 2003;Komatitsch et al, 2004;Lee et al, 2008;Chaljub, 2009;Lee, Chan, et al, 2009;Stupazzini et al, 2009;Chaljub et al, 2010;Tape et al, 2010). In particular, areas characterized by low wavespeed sedimentary basins may show a very different ground response compared to hardrock sites, with strong amplifications, multiple reverberations, and prolonged shaking (e.g., Rovelli et al, 2001;Olsen et al, 2006;Chaljub et al, 2007;Lee et al, 2008).…”

Frequency‐Scaled Curvature as a Proxy for Topographic Site‐Effect Amplification and Ground‐Motion Variability

“…However, the complexity of the phenomenon (i.e., frequency and source dependencies, intrinsic 3D effects) makes it difficult to provide general models able to predict motion patterns in realistic topographic configurations. Recently, 3D numerical simulations of ground motions have been useful to estimate site effects on realistic, intricate topographies (Lee, Chan, Komatitsch, et al, 2009;Chaljub et al, 2010;Maufroy et al, 2012;De Martin et al, 2013). Nonetheless, this kind of sophisticated simulations remains computationally expensive and thus unsuitable for common seismic-hazard studies.…”

Physics-Based Earthquake Ground Shaking Scenarios in Large Urban Areas

“…Two major experiments of verification of such numerical codes were conducted in the second half of the last decade, namely within the ShakeOut (Bielak et al 2010) and the Grenoble (Chaljub et al 2010) benchmarks, while a further experiment is in progress (E2VP) based on the Euroseistest configuration (Chaljub et al 2013). …”

“…The mesh generation may be accomplished using a third party software, e.g. CUBIT (http://cubit.sandia.gov/) The code has been verified over different benchmarks, including that of Grenoble (Chaljub et al 2010), and a further comparison with an independent solution is described in the following.…”