The importance of poroelastic effects in models of injection-induced slip on rate-and-state faults

Shengduo Liu, Elias R. Heimisson, & Nadia Lapusta

Published August 16, 2021, SCEC Contribution #11638, 2021 SCEC Annual Meeting Poster #174

Slip on faults induces compression and dilation in the surrounding rock, which can interact with pore fluids, modify pressure, and induce fluid flow; those fluid effects can, in turn, affect the slip evolution. The coupling of slip and pore fluids is often ignored in modeling of seismic and aseismic slip, although several studies that considered the coupling using poroelasticity found that it can affect the fault slip in qualitative ways (Dunham and Rice, JGR, 2008; Heimisson et al., JMPS, 2019).

Here, we study the poroelastic coupling on a 1D fault that experiences fluid injection in a 2D in-plane bulk. We use a novel spectral boundary-integral approach for quasi-dynamic simulations of slip on a fault embedded into a fluid-saturated and diffusive poroelastic bulk, developed mainly by Elias Heimisson (Heimisson et al., manuscript in preparation). The fault is governed by rate-and-state friction and incorporates simultaneously the effects of inelastic dilatancy and fluid injection on pore fluid pressure. The fluid injection is simulated by increasing pore fluid pressure in a small portion of the fault, using the pressure history of a field experiment (Guglielmi et al., 2015).

Our findings confirm that inelastic dilatancy tends to stabilize fault slip by reducing pore fluid pressure and increasing effective normal stress when slip speed is sufficiently fast such the diffusion into the bulk or along the shear layer cannot equilibrate the pore-pressure change. However, we also find that the effect of poroelastic coupling in the bulk on slip stability can be comparable to that of inelastic dilatancy. For example, for the case with stronger poroelastic effects, i.e. a larger difference between the drained and undrained Poisson’s ratio, significant slow slip occurs later in the same injection process and seismic fault slip nucleates later, when the slipping region is larger. We also find that sufficiently large bulk diffusivity can stabilize fault slip since pore pressure change due to injection is more efficiently equilibrated by diffusion into the surrounding bulk. In the future, we plan to investigate a broad parameter regime and establish conditions under which poroelastic effects cannot be ignored and can, in fact, dominate the slip evolution in response to injection. We also plan to explore different physical representations of the fault shear layer.

Key Words
Earthquake source, poroelastic effects, induced seismicity

Citation
Liu, S., Heimisson, E. R., & Lapusta, N. (2021, 08). The importance of poroelastic effects in models of injection-induced slip on rate-and-state faults . Poster Presentation at 2021 SCEC Annual Meeting.

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