Permeability of underthrust sediments at the Costa Rican subduction zone: Scale dependence and implications for dewatering and fault strength

Sediment permeability is a key parameter controlling fluid overpressures and fluid egress at subduction zones. Data compilations for argillaceous sediments illustrate that scale effects on permeability are generally minor. However, parallel studies of permeability at multiple scales for the same sediments are rare. We report on laboratory permeability measurements on sediments incoming to the Costa Rican subduction zone, at porosities from 26% to 80%. We then apply a method for directly estimating average permeability at the scale of sediment layer thickness, constrained by published estimates of excess pore pressure. Permeability values from both methods are in excellent agreement, and exhibit a trend consistent with permeability-porosity relationships used in numerical models at scales of several to 10's of km. Our results indicate no scale dependence of permeability. The permeability-porosity relationship we derive is comparable to those inferred at the Nankai and Barbados subduction zones, indicating that the high porosity, and hence high permeability, of incoming sediments at Costa Rica is the most likely explanation for comparatively rapid dewatering there. One key implication is that at non-accretionary subduction zones, especially those with thin incoming sediment sections, the high porosity (and thus high permeability) of subducted sediments should contribute to rapid compaction, high volumes of fluid expulsion, better drainage, and higher strength at the wedge base when compared with accretionary systems characterized by thick incoming sediments.