[S-CG45] Science of slow-to-fast earthquakes

[SCG45-25] From seismic to aseismic slip by changing poroelastic and bulk properties during fluid injection

★Invited Papers

*Elias Rafn Heimisson¹, Shengduo Liu², Nadia Lapusta², John Rudnicki³ (1.ETH Zurich Swiss Federal Institute of Technology Zurich, 2.California Institute of Technology, 3.Northwestern University)

Keywords:SEAS, Injection, poroelastic, induced earthquakes, rate-and-state

Faults at seismogenic depths in the crust are often idealized as fractures or discontinuities in an elastic medium. However, a more accurate description would consider the effects of the fluid-saturated porous medium surrounding the fault. The theory of poroelasticity offers a practical mechanical description of the natural fault environment and, simultaneously, allows us to consider the coupling of fluid and solid phases during fault slip in a more rigorous manner. Poroelasticity describes the two-way coupling of a solid porous elastic rock matrix and a fluid phase, which saturates the pore space. If pore pressure changes, the elastic rock matrix is strained, and if the rock is volumetrically strained, the pore pressure changes. A complex interplay of pore pressure in the bulk and shear zone of a fault emerges when we consider the multiple fluid processes that couple to slip. We present a spectral boundary integral method that allows us to simulate slip on a 2D quasi-dynamic rate-and-state fault with fully coupled poroelastic bulk, state-dependent dilatancy, and fluid injection. The method allows for anisotropic permeability and non-linear physics in the shear zone, while the bulk is isotropic and homogenous. We apply the method to understand nucleation and repeated fault ruptures with a realistic pore pressure injection history from a field experiment. We compare different cases where bulk diffusivity and poroelastic properties are changed and compare with in-elastic dilatancy. As expected, by systematically increasing the dilatancy coefficient, we observe a transition from highly unstable seismic slip to a migrating slow slip front to quasi-static slip localized to highly pressurized areas. More surprisingly, we find that differences in drained and undrained poroelastic properties and bulk diffusivity strongly influence fault slip stability. A larger difference between drained and undrained Poisson’s ratio or higher bulk diffusivity results in more stable slip during injection, fewer ruptures, and delayed nucleation. These effects appear to be of comparable importance to dilatancy. In the most unstable simulations, we observe multiple ruptures of the same fault patch that systematically grow larger. We propose that this may represent a way to develop a runaway rupture during injection into faults. That is a rupture that propagates far outside the pressurized region. We conclude that the poroelastic properties of the bulk, which are typically ignored, play a critical role in the stability and determining if slip is seismic or aseismic during injection.