Along-arc variation in short-term slow slip events caused by 3-D fluidmigration in subduction zones

A 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.