Slow slip events (SSEs) are commonly observed at a plate interface between a viscously slipping portion and a seismogenenic zone. Recently, SSEs have also been reported following in-situ fluid injection experiments. Almost all exiting models of SSEs are friction-based, utilizing unstable-stable transitional fault frictional behavior, in conjunction with suitable rheology of fault zones in the presence of hydrostatic pressure or overpressure.

Inspired by the theory of poroelasticity, here we propose a fluid-based alternative model in which spontaneous quasi-static shear dislocation is initiated and sustained by the time-dependent poroelastic stress generated by the transient fluid pressure gradient. This model relies on the key assumption that the fault is critically stressed and has an exceedingly low frictional strength. We test our model via a series of 2D numerical experiments in the framework of coupled poromechanics. We simulate fluid injection into a faulted porous medium by solving the fluid diffusion equation. At each time step, the pressure gradient is passed as an equivalent body force vector to the quasi-static force balance law to solve for the displacement while allowing slip to occur simultaneously on the fault. Therefore, the slip rate is controlled by the fluid diffusion. We designed several cases in which the diffusivity of the matrix and the fault are on the order of 10^-3 m²/s and 10^-4 ~10² m²/s, respectively. Depending also on other parameters including injection pressure and mechanical properties, the modeled dislocation is on the order of 10^-4 ~10^-5 m and the sliding velocity, 10^-7 ~10^-9 m/s, suggesting our model as a promising alternative source model of SSEs.