A spectral boundary-integral method for faults and fractures in a poroelastic solid: Simulations of a rate-and-state fault with dilatancy, compaction, and fluid injection
Elias R. Heimisson, ETH Zürich

Fluid-fault interactions result in many two-way coupled processes across a range of length scales, from the micron scale of the shear zone to the kilometer scale of the slip patch. The scale separation and complex coupling render fluid-fault interactions challenging to simulate and may ultimately limit our understanding of experimental data and induced seismicity. Here we present spectral boundary-integral solutions for in-plane interface sliding and opening in a poroelastic solid. We solve for fault slip in the presence of rate-and-state frictional properties, inelastic dilatancy, injection, and the coupling of a shear zone and a diffusive poroelastic bulk. The shear localization zone is treated as having a finite-width and non-constant pore pressure, albeit with a simplified mathematical representation. The dimension of the 2D plane strain problem is reduced to a 1D problem resulting in increased computational efficiency and incorporation of small-scale shear-zone physics into the boundary conditions. We apply the method to data from a fault injection experiment that has been previously studied with modeling. We explore the influence of inelastic dilatancy, bulk poroelastic response, and bulk diffusivity on the simulated fault slip due to the injection. Dilatancy not only alters drastically the stability of fault slip but also the nature of pore pressure evolution on the fault, causing significant deviation from the standard square-root-of-time diffusion. More surprisingly, varying the bulk's poroelastic response (by using different values of the undrained Poisson's ratio) and bulk hydraulic diffusivity can be as critical in determining rupture stability as the inelastic dilatancy.

https://doi.org/10.1002/essoar.10510458.1

Fluids in earthquake cycle models
Pierre Dublanchet, MINES ParisTech

Fluids are suspected to play an important role in earthquake mechanics, and more generally in tectonic fault slip reactivation. This is suggested by the study of induced seismicity sequences, by the seasonal modulation of earthquake production, or the observation of earthquakes, LFE and slow slip events in possibly hydrated regions of subducting slabs. Recent laboratory and decametric scale hydromechanical experiments have also revealed the contribution of pressurized fluid in fault slip reactivation. 

Fluids therefore need to be accounted for in fault and earthquake cycle models. This could be achieved by coupling thermo-poro mechanics to classical fault models based on elasticity and slip-dependent friction. Here I will discuss how a simplified coupling, based on the concept of effective normal stress and non-linear fluid diffusion, could be used to improve our understanding of fault slip reactivation, and the development of (natural and induced) earthquake swarms. The approach shown here relies on the comparison between simplified hydromechanical models and observations of seismicity and fault slip, from the laboratory to the active fault system scale.

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