Tandem: An Open-Source High-Performance Computing Volumetric Software Package to Model Sequences of Earthquakes and Aseismic Slip Across Complex Fault Systems

Abstract Simulating sequences of earthquakes and aseismic slip (SEAS) on realistic fault systems provides a physical framework to understand the evolution of the seismic cycle, but remains computationally expensive. Volumetric approaches offer the physical flexibility to handle complex geometries and heterogeneous off-fault media but may incur prohibitively high computational costs when applied to the vast range of spatial and temporal scales inherent to earthquake cycles. This article documents developments in Tandem, an open-source volumetric SEAS simulation software package that addresses these challenges using a symmetric interior penalty discontinuous Galerkin formulation on unstructured curvilinear meshes in 2D and 3D with high-order polynomial basis functions. We describe Tandem from a user’s perspective, covering mesh formats, checkpointing, model configuration via human-readable files, and flexible loading schemes for various tectonic settings. To lower barriers to usage, Tandem is distributed as a standalone C++/PETSc code, a preconfigured virtual machine image, and as an application on the Quakeworx Science Gateway, where users can run simulations without installing dependencies or securing direct high-performance computing (HPC) access. We report on practical development choices of interest to SEAS and other scientific software developers. To mitigate the cost of volumetric discretization, the software provides both matrix-free and assembled-matrix formulations with a fully volumetric explicit approach utilizing hybrid geometric–algebraic multigrid preconditioners and a discrete Green’s function mode that accelerates timestepping by precomputing traction kernels. We document performance on both central processing unit and graphical processing unit supercomputers, including near-ideal weak scaling up to 112,000 message passing interface (MPI) ranks on the supercomputer Frontera. Finally, as demonstration examples, we present validation in a 3D SEAS community benchmark (BP7) and a 2D example examining how off-megathrust material heterogeneity influences seismic cycle behavior. By sharing software design choices as well as practical guidance for its use, we hope to make volumetric HPC-driven SEAS modeling more accessible to the earthquake science community.