Hydrodynamic response of subduction zones to seismic activity: A case study for the Costa Rica margin Paula A. Cutillo a, ⁎ , Shemin Ge b , Elizabeth J. Screaton c a National Park Service, Water Resources Division, Fort Collins, Colorado, 80525, USA b Department of Geological Sciences, University of Colorado, Boulder, Colorado, 80309, USA c Department of Geological Sciences, University of Florida, Gainesville, Florida, 32611, USA Accepted 7 February 2006 Available online 30 June 2006 Abstract Thermal anomalies in tectonically active areas are often attributed to sub-seafloor fluid circulation and faulting mechanisms, particularly in subduction zones where the largest thrust earthquakes occur. Postseismic fluid flow is enabled by the poroelastic response of the fault system to the earthquake's strain field, as well as by the rupturing of permeability barriers in the vicinity of the fault zone. We investigated the relative importance of these mechanisms on postseismic pore-pressure diffusion and advective heat transport in the subduction zone setting. A two-dimensional numerical fluid flow and heat transport model was developed for the Costa Rica subduction zone offshore of the Nicoya Peninsula. The flow and transport model was coupled with an earthquake strain model to quantify the effects of coseismic strain and permeability enhancement on fluid pressures and temperatures within the Costa Rica margin. Coseismic changes in pore pressure and postseismic pore-pressure diffusion were found to be sensitive to the compressibility of the porous medium, and patterns of pore-pressure recovery were more complex than that predicted by theoretical faulting models. Coseismic contraction and extension of the crust produced high fluid pressures close to the fault, while the inflow of fluid from depth increased fluid pressures several years following the simulated fault slip. Crustal deformation alone was not observed to perturb the temperature field. Laterally extensive permeability increases of two orders of magnitude along the décollement were required to produce small changes in heat flow. Local permeability changes in the upper slope region of least five orders of magnitude were necessary to noticeably affect heat flow. The results of the numerical simulations may help to refine conceptual faulting models and provide guidance for locating long-term hydrologic monitoring sites at Costa Rica and other subduction zones. © 2006 Elsevier B.V. All rights reserved. Keywords: Costa Rica margin; Subduction zones; Heat transport; Earthquake strain 1. Introduction Evidence of coupling between seismic activity and subsurface fluid flow is illustrated by water-level fluc- tuations in wells (e.g, Roeloffs et al., 1995; Lee et al., 2002) stream-flow fluctuations (e.g., Rojstaczer and Wolf, 1992) and increased fluid flow and temperatures at hydrothermal vents (e.g., Johnson et al., 2000) following earthquake activity. Fluid migration below the seafloor as well as fault-valve mechanisms are commonly in- voked to explain thermal anomalies at convergent mar- gins (e.g., Foucher et al., 1990; LePichon et al., 1990; Silver et al., 2000). In subduction zones, sub-seafloor fluid pressures and circulation may influence seismicity Tectonophysics 426 (2006) 167 – 187 www.elsevier.com/locate/tecto ⁎ Corresponding author. Tel.: +1 970 225 3537. E-mail addresses: paula_cutillo@nps.gov (P.A. Cutillo), ges@colorado.edu (S. Ge), screaton@ufl.edu (E.J. Screaton). 0040-1951/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2006.02.017