Partial ruptures governed by the complex interplay between geodetic slip deficit, rigidity, and pore fluid pressure in 3D Cascadia dynamic rupture simulations

Jonatan Glehman, Alice-Agnes Gabriel, Thomas Ulrich, Marlon D. Ramos, Yihe Huang, & Eric O. Lindsey

Published September 8, 2024, SCEC Contribution #13797, 2024 SCEC Annual Meeting Poster #034 (PDF)

Physics-based simulations are crucial to assessing the seismic hazard in the Cascadia subduction zone (CSZ), requiring assumptions about fault stress and material properties. Geodetic slip deficit models (SDMs) may inform the initial stresses governing megathrust earthquake dynamics. We present a unified workflow linking SDMs to 3D dynamic rupture simulations, and 22 rupture scenarios to unravel the dynamic trade-offs of assumptions on SDMs, rigidity, and pore fluid pressure. We find that most dynamic ruptures arrest before they can propagate along the entire margin. Margin-wide rupture requires a large slip deficit in the central CSZ. Comparisons between Gaussian and smoother, shallow-coupled SDMs show significant differences in stress distributions and rupture dynamics. Variations in depth-dependent rigidity cause competing effects, particularly in the near-trench region. Higher overall rigidity can increase fault slip but also result in lower initial shear stresses, inhibiting slip. The state of pore fluid pressure is crucial in balancing the SDM-informed initial shear stresses with realistic dynamic rupture processes, especially assuming small recurrence time scaling factors. This study highlights the importance of self-consistent assumptions on rigidity and initial stresses between geodetic, structural, and dynamic rupture models, providing a foundation for future simulations focusing on ground motions and tsunami generation. Partial ruptures being favored along the Cascadia margin may suggest that the dynamic conditions conducive to margin-wide ruptures are different from those required for partial ruptures. Our updated framework for estimating the initial stress conditions and careful consideration of how rigidity, pore fluid pressure, and SDMs interplay corroborate the observed tendency for Mw<9 events. However, margin-wide rupture is only realized if the slip deficit in the central CSZ exceeds 10m. Our results suggest prioritizing the reconciliation of the mechanical, frictional, and stress conditions in the central CSZ, as its state exerts first-order control on rupture dynamics and, consequently, tsunamigenesis or strong ground motion. Preliminary ground motion analysis shows that models assuming higher rigidity result in larger peak ground velocity compared to the low-rigidity scenario and are more consistent with the ASK14 empirical ground motion model. At some stations, this amplification exceeds a factor of two.

Key Words
Partial ruptures, dynamic trade-offs of assumptions on slip deficit models, rigidity, and pore fluid pressure, ground motions

Citation
Glehman, J., Gabriel, A., Ulrich, T., Ramos, M. D., Huang, Y., & Lindsey, E. O. (2024, 09). Partial ruptures governed by the complex interplay between geodetic slip deficit, rigidity, and pore fluid pressure in 3D Cascadia dynamic rupture simulations. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology