Breaking the elastic mold: The effect of upper plate permanent deformation on earthquake cycles

Bar Oryan, Alice-Agnes Gabriel, & Dave A. May

Published September 8, 2024, SCEC Contribution #13792, 2024 SCEC Annual Meeting Poster #132

One of the most commonly held assumptions in earthquake hazard assessment is that the off-fault deformation associated with the earthquake cycle is purely elastic. In subduction settings, this suggests that all off-fault strain associated with the slow interseismic loading period is released during large megathrust earthquakes, amounting to effectively zero deformation and surface displacement over numerous cycles. However, recent geomorphological evidence (Oryan et al., 2024) suggests that interseismic stresses can induce increments of irreversible brittle failure across the overriding plate, indicating that a significant portion of interseismic elastic energy is not available to drive megathrust earthquakes.

The impact of this behavior on the coseismic period and earthquake rupture processes remains elusive. To investigate the potential imbalance in co-, post-, and interseismic strain of earthquake cycles, we use Tandem (Uphoff et al., 2022), an open-source code freely available for simulations on the QuakeWorx Science Gateway. Tandem employs the discontinuous Galerkin volumetric solver to model quasi-dynamic sequences of earthquakes and aseismic slip (SEAS). Using Tandem, we simulate SEAS along a 10-degree dipping megathrust fault, modulated by two scenarios mimicking upper plate zone of weakness due to brittle failure. First, we use a population of discrete, randomly distributed upper plate faults adhering to rate and state friction laws. In the second scenario, we examine the effect of a localized zone of reduced rigidity off-fault which serves as a proxy for damage.

Our preliminary findings imply that upper plate faults fail interseismically above the transition in fault coupling, where stresses and permanent deformation are most pronounced. We also find that reduced rigidity may prompt interseismic deformation in shallow velocity-strengthening zones, potentially leading to slip up to the trench, as seen in the Japan Trench. Our study has important implications on how varying increments of irreversible upper plate strain, which reduce the overall elastic strain available to drive interface slip, affect the timing, size, and deformation associated with large megathrust earthquakes.

Key Words
Earthquake cycles, inelastic off fault deformation, bridging long-term observations with short-term cycles

Citation
Oryan, B., Gabriel, A., & May, D. (2024, 09). Breaking the elastic mold: The effect of upper plate permanent deformation on earthquake cycles. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)