Along‐arc variation in short‐term slow slip events caused by 3‐D fluid migration in subduction zones

Abstract

AbstractA strong correlation exists between the average slip rate by short‐term slow slip events (SSEs) and changes in the slab geometry in Cascadia and Nankai. The generation of short‐term SSEs is generally assumed to be related to the presence of fluids and we investigate the hypothesis that fluids released by metamorphic dehydration reactions migrate in 3‐D due to complex slab geometry. The associated along‐arc focusing of fluid flux is likely to cause higher average slip rate in certain patches. To test this hypothesis, we investigate how fluid migration is modified by along‐strike changes in slab geometry. We use a numerical model of two‐phase flow in subduction zones. In this model fluids migrate subparallel to the slab surface due to the anisotropic permeability inside a serpentinite layer just above the slab. In 3‐D, we find that fluids migrate in the maximum‐dip direction of the slab, rather than subparallel to the plate motion. As a result fluid paths concentrate with increasing porosity where the slab has a convex shape (and diverge with decreasing porosity where it has a concave shape). These results suggest that regions with a high average slip rate by short‐term SSEs in Cascadia and Nankai can be explained by 3‐D focusing of fluid migration. We predict a defocusing of fluids below the Kii Channel, Nankai, which may be the reason for the observed small slip by short‐term SSEs in this location.