Observation and Modeling of Seafloor Flow Rate Response to Slow Slip Deformation at the Costa Rica Forearc
Observations of transient flow through the seafloor during the 2000 Costa Rica Seismogenic Zone Experiment off the Nicoya peninsula suggests transient deformation events occurred in the shallow subduction zone, around 10 km arcward from the Middle America Trench. The observed deformation in this region is unexpected, as it is generally believed that accommodation of plate convergence in the shallow plate interface is through stable sliding. This study is an investigation of the type and extent of deformation events that could cause these flow transients. Fluid flow rates in response to displacement on a fault in a porous media are numerically calculated using a fully- coupled poroelastic finite element model. Modeled flow rates at the surface of a half-space, the seafloor, are intended to assist in interpretation of records from seafloor flow meter instrumentation. Model results show seafloor fluxes are a superposition of two effects causing volumetric strain at the surface: 1) compressional and dilational regions that radiate out from the fault tips, and 2) extensional and compressional bending of the free- boundary surface. Solutions of the spatial and temporal flow rate response to sudden slip along the decollement of a subduction zone are presented for near-field to far-field ruptures in a homogeneous crust and a heterogeneous crust with a layer of sediment overlying the basement. The characteristic patterns in flow rate for near-field and far-field ruptures in homogeneous and heterogeneous crust help to determine optimal instrument placement for future subduction zone studies. In addition, comparisons of the fully-coupled half-space model simulations for a homogeneous verses a heterogeneous crust demonstrates situations where it is safe to use existing analytical solutions for finite ruptures in a homogeneous poroelastic half-space instead of finite element modeling. Novel simulations of updip and downdip propagating ruptures in this study demonstrate unique temporal records of flow rate through the seafloor surface. These propagating rupture model results are applied to help constrain rupture characteristics of a flow event recorded on the toe of the Costa Rica prism. The observed flow rate time series is nicely reproduced with a downdip propagating rupture centered below a flow meter instrument 6.5 km arcward from the trench. The observed variability in flow rates recorded at 2 instruments located at along-strike distances of 15 and 30 km could be explained by a single bilaterally propagating event that has an along-strike variation in rupture initiation. This is the first result suggesting episodic slow slip may initiate near the trench and possibly independent of a triggering event further downdip. A shallow slow release of stored energy is also the likely cause for shallow propagation during the nearby 1992 Nicaragua tsunamogenic earthquake. This proposed slow slip event in the frontal Nicoya prism, a section where subducted clay-rich sediments along the decollement are saturated and hydrous, poses important questions concerning processes controlling temporal variation of fault mechanics in the 'stable-sliding' prism toe. How does stress accumulate at the shallow plate interface? Perhaps the topography associated with the normal faulting of the subducting basement prevents stable sliding from occurring until a critical stress threshold is overcome.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 8100 TECTONOPHYSICS