3-D numerical simulations of earthquake ground motion in sedimentary basins: testing accuracy through stringent models

Abstract

Differences between 3-D numerical predictions of earthquake ground motion in the Mygdonian basin near Thessaloniki, Greece, led us to define four canonical stringent models derived from the complex realistic 3-D model of the Mygdonian basin. Sediments atop an elastic bedrock are modelled in the 1D-sharp and 1D-smooth models using three homogeneous layers and smooth velocity distribution, respectively. The 2D-sharp and 2D-smooth models are extensions of the 1-D models to an asymmetric sedimentary valley. In all cases, 3-D wavefields include strongly dispersive surface waves in the sediments. We compared simulations by the Fourier pseudo-spectral method (FPSM), the Legendre spectral-element method (SEM) and two formulations of the finite-difference method (FDM-S and FDM-C) up to 4Hz. The accuracy of individual solutions and level of agreement between solutions vary with type of seismic waves and depend on the smoothness of the velocity model. The level of accuracy is high for the body waves in all solutions. However, it strongly depends on the discrete representation of the material interfaces (at which material parameters change discontinuously) for the surface waves in the sharp models. An improper discrete representation of the interfaces can cause inaccurate numerical modelling of surface waves. For all the numerical methods considered, except SEM with mesh of elements following the interfaces, a proper implementation of interfaces requires definition of an effective medium consistent with the interface boundary conditions. An orthorhombic effective medium is shown to significantly improve accuracy and preserve the computational efficiency of modelling. The conclusions drawn from the analysis of the results of the canonical cases greatly help to explain differences between numerical predictions of ground motion in realistic models of the Mygdonian basin. We recommend that any numerical method and code that is intended for numerical prediction of earthquake ground motion should be verified through stringent models that would make it possible to test the most important aspects of accuracy.

Earthquake Ground Motion in the Mygdonian Basin, Greece: The E2VP Verification and Validation of 3D Numerical Simulation up to 4 Hz

“…In conclusion of the verification phase, the encouraging level of agreement among numerical predictions up to 4 Hz, the subsequent identification of the factors affecting the accuracy of the simulations, and indications for correct handling of those factors (Chaljub et al, 2015) support the use of the 3D numerical modeling approach for predicting ground motion, at least in the linear, low-to-intermediate frequency range, and provided it is performed wisely and carefully.…”

“…All of those key elements need proper implementation in the methods and codes for a sufficiently accurate simulation of ground motion at sites atop complex local sedimentary structures. A more detailed discussion of these factors and the way to reduce the related code-to-code differences are provided by Chaljub et al (2015).…”

“…The article is accompanied by the methodological study of Chaljub et al (2015). It focuses on quantitative and qualitative analysis of accuracy (i.e., verification) of four numerical modeling methods in their application to stringent canonical models directly related to the model of the Mygdonian basin.…”

Sensitivity analysis of synthetic seismograms in sedimentary basin with respect to uncertain seismological parameters

“…Our implementation is being widely used for computing seismic wave propagation in complex geological media (De Martin et al 2013;Matsushima et al 2014;Chaljub et al 2015;Maufroy et al 2015). An animation of wave propagation performed for this study is available at http://efispec.free.fr/videos/e2vp2.shaheenII.mp4.…”

“…To foster the use of linear 3D numerical simulations in practical prediction, E2VP aimed at (a) evaluating the accuracy of the most-advanced numerical methods when applied to realistic 3D models and (b) providing an objective, quantitative comparison between recorded earthquake ground motions and their numerical predictions. So far, these two objectives (a) and (b) have been thoroughly tackled; they are published in Chaljub et al (2015) and Maufroy et al (2015). As a continuation of the E2V project, the sourcerelated variability of the site is presented by Maufroy et al 2017; and this study investigates the variability and sensitivity of earthquake ground motions associated with the epistemic uncertainties of the E2VP model.…”