Consecutive Flow Events and Propagating Aseismic Slip at the Costa Rica Forearc: Matching Models to Measurements

Of Chemical Aqueous Transport (CAT) meters placed at the Costa Rica margin Dec. 1999 - Jan. 2000, three meters simultaneously recorded three flow events (Brown et. al. in press). With no seismic activity, but rather a seismic noise record that correlates with fluid pulses, we hypothesize these fluid events are a result of strain imposed by aseismic deformation. This theory is tested using a 3-d finite element modeling package to simulate transient slip events and subsequent poroelastic relaxation. The osmotically driven CAT meters developed at Scripps Hydrogeology and Tectonics Studies Laboratory are capable of detecting both inflow and outflow from the sedimented seafloor at Darcy flow rates of ~10-5 cm/d to 3 cm/d. With its cost efficient design, it was possible to deploy fourteen flow meters with 15 km grid spacing to constrain the flow field over an approximate 50 by 70 km section of the forearc. The three CAT meters located in the out-of-sequence thrust (OOST) zone spanned an along-margin strike distance of 30 km, were 7-12 km from trench, and ~1-1.5 km above the decollement according to a seismic survey. A sequence of three flow events measuring a net flow of 2-6mm and lasting 2-3 weeks was recorded on all three of these instruments at the forearc toe. Amplified sensitivity at one co-located OBS of these three instruments recorded a significant increase in noise averaged over a 12 hour period coinciding with timing of flow events at all three OOST sites. We hypothesize that a strain related to an aseismic event is causing pore pressure variations observed surface fluid flow records. Model simulations help to determine if reasonable dislocation, depth, rupture dimension of aseismic slip, and poroelastic constants for post slip relaxation can generate the flow record captured at the surface. We start by building a realistic representation of geometry at the Costa Rica subduction zone, and using of poroelastic parameters of the sediment and basement determined from bore hole studies. A series of simulations testing the fluid response to aseismic slip events of different size and location are then run in attempt to match transient flow signal collected over the array of flow rate measurements at the Costa Rica forearc. Feasibility of parameter sets generating response in kind to empirical measurements will be presented.