FLUID FLOW AND SEISMIC DEFORMATION IN SUBDUCTION ZONES

Strains resulting from dislocations along fault surfaces perturb the fluid flow field, leading to co-seismic fluid pressure anomalies and triggering subsequent transient fluid diffusion. We examine the processes of seismic deformation and fluid diffusion to gain insights into the hydrologic and mechanical interactions at plate boundaries. Results have implications to upcoming investigations of the Nankai and Costa Rica subduction zones. The Subduction zones, typically consisting of marine sediments, a decollement zone, and the upper oceanic crust, are characterized by heterogeneous mechanical and hydrogeological properties. Accordingly, distributions of seismic strain and pore pressure response are expected to vary. Using a numerical modeling approach, the effects of heterogeneity are investigated by assigning different mechanical and hydrological properties to different lithologic zones. Modeling suggests that for a reasonable range of parameter scenarios, transient pressure head signals caused by discrete dislocations of a few meters in the updip region of the seismogenic zone can be observed over large areas of the margin from within shallow depths of the sediment wedge to the oceanic crust below the decollement. Modeling of fluid and heat transport reveals that seismic deformation combined with laterally extensive permeability increases in the décollement is required to produce observable heat flow anomalies. Significant permeability changes in localized regions near the seafloor also contribute to noticeable changes in heat flow. We further investigate the effects of progressive fault slip on pore pressure records. Modeling results suggest that pore pressure records from monitoring wells have the potential to provide information on the nature of a variety of slip processes including aseismic slip, slow slip, and very low frequency seismicity. Modeling further suggests that clay- and silt- rich sediments are capable of providing good records of pore pressures in response to progressive slip, because of their expected low hydraulic diffusivity. In contrast, sand-rich sediments or permeable basaltic crust would be poor sites for recording progressive slip caused pore pressure changes because flow would rapidly modify the pore pressure signal.