The Complex Interplay Between Pore Fluid Pressure, Rupture Dynamics, and Fault Mechanics in the Cascadia Subduction Zone: Insights from 3D Dynamic Rupture Simulations

Jonatan Glehman, & Alice-Agnes Gabriel

Published September 10, 2023, SCEC Contribution #12998, 2023 SCEC Annual Meeting Poster #109 (PDF)

Pore fluid pressure (Pf) in subduction zones may play a significant role in governing the dynamics of megathrust earthquakes. While, in some regions, Pf may be approximated by observations, the state and potential variability of Pf at the Cascadia Subduction Zone (CSZ) remains debated. High Vp/Vs ratios observed in the CSZ can be explained by high (near-lithostatic) Pf (Audet et al., 2009). This is consistent with the assumption that mature faults are effectively frictionally weak. However, recent consolidation analysis (Tobin, AGU '22) implies a strong wedge environment and high seismic velocity with close-to-no fluid overpressure (hydrostatic conditions).

We perform 3D dynamic rupture and seismic wave propagation simulations using SeisSol to study the impact of low (hydrostatic), high (nearly lithostatic), or along-strike variable Pf on earthquake nucleation, propagation, and arrest on the CSZ. To do so, we (1) convert and interpolate a Gaussian distribution locking model (Schmalzle et al., 2014) into heterogeneous initial stresses on the Slab2.0 geometry, following Ramos et al. (2021). However, here, using SeisSol in a pseudo-static simulation step, minimizing interpolation challenges and unifying the workflow of linking geodetic locking models to dynamic rupture simulations. (2) Modify depth-dependent effective normal stresses (σ'_n) to account for variable Pf. Our preliminary findings indicate that high Pf (corresponding to low σ'_n) leads to a larger total slip on the fault compared to low Pf (corresponding to high σ'_n) and higher slip to the trench, with important implications for regional seismic and tsunami hazards. Moving forward, we aim to quantify the effect of high Pf (Vyas et al., 2022) by computing ground motion intensities (e.g., PGA, PGV, PSA) using a 3D velocity model. Subsequently, we will calculate the amplification factors with respect to the scenario with low Pf and compare the intensities to the NGA-West2 ground motion models. Our models will be readily extendable to newly compiled observational data informing on the CSZ. The outcome of our study will highlight the importance of numerical simulation as a complementary tool for seismic hazard assessments and earthquake risk mitigation in regions with similar tectonic settings where data is sparse and seismic activity is low.

Key Words
3D numerical simulations, Cascadia subduction zone, Pore fluid pressure, rupture dynamics, seismic and tsunami hazards

Citation
Glehman, J., & Gabriel, A. (2023, 09). The Complex Interplay Between Pore Fluid Pressure, Rupture Dynamics, and Fault Mechanics in the Cascadia Subduction Zone: Insights from 3D Dynamic Rupture Simulations. Poster Presentation at 2023 SCEC Annual Meeting.

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