Effects of Bulk Poroelasticity on Repeating Earthquakes: Implications from Numerical Modeling

Linxuan Li, & Nadia Lapusta

Published September 8, 2024, SCEC Contribution #13806, 2024 SCEC Annual Meeting Poster #133

Most earthquake source studies ignore poroelastic effects activated by fault slip, in which bulk pore-fluid pressure (PFP) evolves due to shear-induced volumetric strain changes. We employ a spectral boundary integration method (Heimisson et al., JGR, 2022) to simulate repeating earthquake sequences on a 1D fault embedded in a 2D poroelastic bulk. The fault is treated as a permeable shear layer with a (small) finite width and piecewise-linear pore fluid pressure distribution, which allows us to consider fluid diffusion across and along the layer. The shear resistance of the fault is given by a rate-and-state friction coefficient times effective normal stress. The relevant PFP to include in the effective normal stress depends on slip-localization assumptions: Using the width-averaged (aka mean) PFP, maximum PFP across the layer, or PFP on one side of the layer corresponds to assuming, respectively, uniform slip across the layer, slip localization at the location of maximum PFP, and slip localization between the layer and bulk as often occurs in nature and experiments.

We find that poroelastic effects significantly modify the recurrence time of repeating earthquakes and the ratio of seismic slip to total potency. Using the mean PFP in the effective stress yields result close to the undrained limit that can be reproduced by elastic simulations with undrained (larger) values of elastic moduli. Such simulations result in a stabilizing effect compared to elastic simulations with regular, drained elastic moduli. Considering the maximum PFP causes additional weakening that promotes unstable fault slip, an effect that competes with the stabilizing effect of the undrained elastic moduli, generating high seismic potency. Using PFP on one side of the shear layer results in more complex ruptures and lower seismic potency. We will report on our current efforts to quantify the poroelastic effect by comparing it to elastic simulations with different Poisson's ratios and frictional properties. Our results suggest that it is important to (i) include poroelastic effects if we would like to use repeating-earthquake observations to constrain fault properties and (ii) understand whether and how slip localizes in the shear layer as the associated fluid effects significantly depend on that assumption.

Citation
Li, L., & Lapusta, N. (2024, 09). Effects of Bulk Poroelasticity on Repeating Earthquakes: Implications from Numerical Modeling. Poster Presentation at 2024 SCEC Annual Meeting.

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