Hydrologic detection and finite element modeling of a slow slip event in the Costa Rica prism toe

We investigate transient fluid flux through the seafloor recorded near the Costa Rica trench during the 2000 Costa Rica Seismogenic Zone Experiment using a 2-D fully coupled poroelastic finite element model. We demonstrate that the observed hydrologic anomalies are consistent with a model of propagating slow slip at the subduction interface between the frontal prism and downgoing plate. There are two sources of volumetric strain that drive fluid flux at the seafloor in response to fault slip at depth: (1) compression and dilation in the vicinity of the tips of a slipping patch and (2) extension and compression due to flexure of the seafloor. The superposition of these two effects results in distinctive spatial and temporal patterns of fluid flow through the seafloor. In a forward modeling approach, time series from shear ruptures with a range of fault length-to-depth ratios in a heterogeneous crust are generated and compared with flow rate observations. Assuming a constant propagation rate and an elliptical profile for the distribution of slip along the decollement, the set of model predictions enables us to infer the probable rupture location, extent, propagation velocity, and duration from a single flow rate time series. The best fit model suggests that the slow slip event initiated within the toe at a depth of less than 4 km and propagated bilaterally at an average rate of 0.5 km d(-1). This interpretation implies that stress in the shallow subduction zone is relieved episodically. Furthermore, the Costa Rica data suggest that episodic slow slip events may initiate in the prism toe without being triggered by a seismic event further downdip.