[S-CG44] Science of slow-to-fast earthquakes

[SCG44-19] Properties of slow slip events explained by velocity-strengthening faults subject to pore fluid pressure perturbations

*Andrea Perez¹, Yoshihiro Kaneko², Martha Savage¹ (1.Victoria University of Wellington, 2.Kyoto University)

Keywords:pore fluid pressure, numerical simulations, slow slip events

Geophysical observations have suggested temporal pore fluid pressure changes that correlate with the occurrence of slow slip events (SSEs) in Hikurangi, Cascadia and Nankai subduction zones (Warren-Smith et al. 2019, Gosselin et al. 2020, Nakajima et al. 2018, Kita et al. 2021). These fluctuations in pore fluid pressure are interpreted as due to fluid migration during and between SSE cycles, which could be promoted by permeability changes during SSE's slip. To examine the role of pore fluid pressure changes on SSEs, we use numerical simulations to calculate the effect of periodic perturbations of pore fluid pressure on a velocity-strengthening (VS) rate-and-state fault. We assume two types of pore pressure perturbations to approximate the observed pore fluid pressure changes: Type I perturbation assumes a Gaussian spatial distribution of pore fluid pressure in both time and space, while Type II assumes fluid injection and along fault fluid diffusion. We find that SSEs are triggered by both perturbation types in VS faults. The properties of the triggered SSEs depend mainly on the relative amplitude, length scale and duration of the perturbation. By varying these controlling parameters, we find that our model is able to reproduce a broad range of SSE properties, including those of both short-term and long-term SSEs. Remarkably, two models (one for each perturbation type) with periodic pore fluid pressure changes of a few MPa reproduce the properties (i.e. duration, slip, magnitude and recurrence interval) of shallow SSEs observed along the northern portion of the Hikurangi margin. Our results indicate that large hydraulic diffusivity values (10 to 10² m²/s) are needed to reproduce the observed properties of shallow Hikurangi SSEs, which suggests high permeability values, in the order of 10^-12 m², in the shear zone where SSEs occur. Such high values could be due to transient and localized increases in fault zone permeability that may result from the opening of fractures during SSEs' slip. Our results suggest that a VS fault subject to periodic pore fluid pressure perturbations is a viable mechanism for the generation of SSEs, which implies that fluids may play a more active role in the generation of SSEs than previously assumed.