An equivalent linear approach to material nonlinearity in regional simulations on a reduced domain

Feiruo Xia, David B. McCallen, & Arben Pitarka

Submitted September 7, 2025, SCEC Contribution #14648, 2025 SCEC Annual Meeting Poster #TBD

EQSIM is a multidisciplinary framework for fault-to-structure earthquake simulations that was developed
as part of the DOE’s Exascale Computing Project (ECP). At the heart of EQSIM is the fourth order accurate finite difference code SW4 (Seismic Waves, 4th Order), which is designed to compute linear wave propagation in kilometer-scale models. SW4 has been used to generate realistic, high fidelity ground motions for large magnitude earthquakes in seismically significant and high population zones such as the Los Angeles basin and the San Francisco Bay Area. However, in these regions in which the geology is characterized by shallow sedimentary basins, large ground deformations caused by basin amplification of seismic waves have the potential to incite non-negligible, nonlinear ground response. In the pursuit of increasing the accuracy of simulated ground motions, it has therefore become of interest to expand the capabilities of SW4 into representing the cyclical softening of soils due to large magnitude earthquake loading.

Here, we present an overview of ongoing work on developing and implementing a computationally feasible nonlinear soil model in SW4. The Equivalent Linear (EQL) method is a simple method of approximating nonlinear soil behavior by iteratively updating the soil shear stiffness and attenuation used in linear simulations. Although the EQL method has been proven to be accurate and computationally efficient while requiring a small number of input parameters, the projected computational cost of iterating on up to 200-300 billion grid points in the regional scale simulations of SW4 remains unfeasible. Therefore, we combine EQL with the domain reduction method (DRM) to allow us to extract from the full regional model only the area where we expect the bulk of the nonlinear response to be – namely, the top one to two kilometers of the near surface.

We expect that this development will enhance simulation capabilities through better representation of the ground motion amplitude and frequency content changes associated with nonlinear response. This will advance EQSIM’s ability to assess earthquake risk, particularly for critical infrastructure founded on the soft soil sites that experience the most nonlinear behavior during large earthquakes.

Key Words
domain reduction method, equivalent linear method

Citation
Xia, F., McCallen, D. B., & Pitarka, A. (2025, 09). An equivalent linear approach to material nonlinearity in regional simulations on a reduced domain. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motions (GM)