Incorporating Full Elastodynamic Effects and Dipping Fault Geometries in Community Code Verification Exercises for Simulations of Earthquake Sequences and Aseismic Slip (SEAS)

Abstract

Numerical modeling of earthquake dynamics and derived insight for seismic hazard relies on credible, reproducible model results. The SEAS (Sequences of Earthquakes and Aseismic Slip) initiative has set out to facilitate community code comparisons, and verify and advance the next generation of physics-based earthquake models that reproduce all phases of the seismic cycle. With the goal of advancing SEAS models to robustly incorporate physical and geometrical complexities, here we present code comparison results from two new benchmark problems: BP1-FD considers full elastodynamic effects and BP3-QD considers dipping fault geometries. Eight modeling groups participated in each benchmark, allowing us to explore these physical ingredients across multiple codes and better understand associated numerical considerations. We find that numerical resolution and computational domain size are critical parameters to obtain matching results, with increasing domain-size requirements posing challenges for volume-based codes even in 2D settings. Codes for BP1-FD implemented different criteria for switching between quasi-static and dynamic solvers, which require tuning to obtain matching results. In BP3-QD, proper remote boundaries conditions consistent with specified rigid body translation are required to obtain matching surface displacements. With these numerical and mathematical issues resolved, we obtain good agreement among codes in long-term fault behavior, earthquake recurrence intervals, and rupture features of peak slip rates and stress drops for both benchmarks. Including full inertial effects generates events with larger slip rates and rupture speeds compared to the quasi-dynamic counterpart. For BP3-QD, both dip angle and sense of motion (thrust versus normal faulting) alter ground motion on the hanging and foot walls, and influence event patterns, with some sequences exhibiting similar-sized characteristic earthquakes, and others exhibiting several earthquakes of differing magnitudes. These findings underscore the importance of considering full dynamics and non-vertical dip angles in SEAS models, as both influence short and long-term earthquake behavior, and associated hazards.

A High‐Order Accurate Summation‐By‐Parts Finite Difference Method for Fully‐Dynamic Earthquake Sequence Simulations Within Sedimentary Basins

“…We verify that both solution methods convergence at their theoretical rates of accuracy by using the method of manufactured solutions. In addition, our code is verified by the community code benchmark problems of Erickson, Jiang, et al (2022), ensuring that our solutions are robust.…”

“…If two different thresholds are used to switch from the coseismic to interseismic solver, it could be that the initial conditions produced for the subsequent dynamic rupture are similar enough that the qualitative characteristics of the rupture are nearly identical. For instance the different modeling simulations included in the community code verification tests of Erickson, Jiang, et al (2022) involved different switching criteria which we found had little impact on long term qualitative rupture characteristics for a homogeneous problem similar to the one we consider in this work. We also find that the switching criteria for the homogeneous version of our problem (i.e., without the sedimentary basin) has little impact on the long term qualitative characteristics and we do not observe long-term divergence using different thresholds.…”

“…Sequences of Earthquakes and Aseismic Slip (SEAS) simulations on the other hand, make a compromise between dynamic rupture simulations and earthquake simulators, modeling long-term earthquake sequences on regional spatial scales with sufficient grid resolution, and incorporating increasingly more rigorous physics such as full dynamic effects (Erickson, Jiang, et al, 2022). Such cycle simulations not only present a method for better understanding the physics of nucleation, but explicitly simulate the stress conditions prior to large earthquakes, and uncover the long term behavior of earthquake processes over multiple cycles.…”

A High-order Accurate Summation-by-Parts Finite Difference Method for Fully-dynamic Earthquake Sequence Simulations within Sedimentary Basins

“…If two different thresholds are used to switch from the coseismic to interseismic solver, it could be that the initial conditions produced for the subsequent dynamic rupture are similar enough that the qualitative characteristics of the rupture are nearly identical. For instance the different modeling simulations included in the community code verification tests of Erickson, Jiang, et al (2022) involved different switching criteria which we found had little impact on long term qualitative rupture characteristics for a homogeneous problem similar to the one we consider in this work. We also find that the switching criteria for the homogeneous version of our problem (i.e.…”

Influence of Creep Compaction and Dilatancy on Earthquake Sequences and Slow Slip

“…During the fully dynamic phase, we continue to enforce the friction law on the fault and free surface condition on the top boundary, but we switch at a nonreflecting condition on the side and bottom boundaries, consistent with the radiation condition that permits outgoing waves. We use a sufficiently large simulation domain to ensure that the solution is relatively insensitive to L y and L z , as explored by Erickson et al (2022).…”