Tandem: A discontinuous Galerkin method for sequences of earthquakes and aseismic slip on multiple faults using unstructured curvilinear grids

Seismic cycle modeling has advanced to the point where 3D models could be connected to geodetic and seismic observations to test hypotheses about controls on the depth and along-strike extent of ruptures, and interactions between seismic and aseismic slip events in geometrically complex fault systems. Such an undertaking does however require a greater degree of geometrical flexibility, material behaviors, code performance, and community participation than has so far been the standard.In this light we present tandem, an open-source C++ code for 3D earthquake sequence modeling (Uphoff et al., 2022a). Tandem solves the elasticity problem with a discontinuous Galerkin finite element method on unstructured tetrahedral meshes. It can handle multiple, nonplanar faults and spatially variable elastic properties (Figure 1). The method can be extended to nonlinear off-fault material response (e.g., power-law viscoelasticity). The code is parallelized with MPI and uses the PETSc-TAO library (Balay et al., 2022) for time-integrators and preconditioned Krylov methods to solve the static elasticity problem. Faults are governed by rate- state friction and adaptive time-stepping permits modeling of dynamic rupture, the postseismic period, and interseismic loading, all across multiple earthquake cycles. The code is developed with best practices for open-source community software and includes documentation and tutorials to facilitate use by the community (github.com/TEAR-ERC/tandem).Uphoff, C., D. A. May, and A.-A. Gabriel (2022a), A discontinuous Galerkin method for sequences of earthquakes and aseismic slip on multiple faults using unstructured curvilinear grids, Geophysical Journal International.Balay, S., et al. (2022), PETSc/TAO users manual, Tech. rep., Argonne National Lab., Argonne, IL, USA (petsc.org/).