Beyond elasticity in ground motion simulations

Steven M. Day

Submitted August 15, 2019, SCEC Contribution #9854, 2019 SCEC Annual Meeting Talk on Sun 1800

Traditionally, numerical simulation of strong motion has been based upon a conceptualization of ground motion as a product of separate source, path, and site factors. This approximation is often a very useful fiction for earthquakes of modest magnitude, and even advanced applications such as CyberShake invoke it to good effect. Over the past 15 years, however, computational resources have grown sufficiently to enable SCEC scientists (and others) to simulate large (>M7) southern California earthquakes at high resolution (i.e., sufficient to resolve sedimentary basin structure at sub-km scale). As a result, it has become clear that this decoupled source/path/site viewpoint is untenable for large events in complex geological environments such as southern California. Instead, simulations strongly suggest that structural complexity, rupture dynamics, propagation path and local site response can couple strongly, both linearly and nonlinearly. A consequence is that nonlinear rock deformation may significantly impact ground motions in ways that differ fundamentally from the traditional engineering understanding of nonlinearity as a shallow site response. On a practical level, this emerging understanding has a number of implications: (i) Nonlinearity may modify ground motion amplitudes at much longer period than previously supposed. (ii) Forward directivity in large events is significantly modified by path complexity as well as by both near-source and along-path nonlinearity, and these effects require a fully coupled treatment. (iii) Nonlinearity in numerical models moderates anomalous extremes of ground motion predictions and can improve the robustness of our predictive capability. (iv) Simulation methodologies for which a linear source/path/site decomposition is fundamentally baked in, such as CyberShake, may need to reassess their treatment of large events. CyberShake, in particular, is limited elastodynamic simulations due to its site-oriented Green’s tensor computational approach. A possible path forward would be to perform fully nonlinear simulations for those events that are flagged as having anomalously large residuals in CyberShake disaggregations.

Key Words
ground motion, nonlinearity, numerical simulations

Citation
Day, S. M. (2019, 08). Beyond elasticity in ground motion simulations. Oral Presentation at 2019 SCEC Annual Meeting.


Related Projects & Working Groups
Ground Motions