HR: 16:50h
AN: S54A-04    [Abstracts]
TI: Deformation of the Cascadia Forearc in Western Oregon
AU: * Verdonck, D
EM: dverdonck@eglab.org
AF: Eastern Geodynamics Lab, 60 Larchmont Rd, Elmira, NY 14905
AB: Deformation across the Cascadia forearc is believed to be a combination of regional strain, semi-rigid block rotation, and localized subduction related strain. In this study, 30 published GPS vectors and 17 leveling lines are used to determine the parameters needed to describe the three modes of deformation in western Oregon. The GPS velocities are relative to stable North America and the sites are all located within the forearc. The total deformation is calculated by superimposing the regional strain, block rotation, and dislocation style deformation. Sixteen parameters describe the dislocation model. The dislocation surface consists of 15 fault segments along a plate interface with non-constant dip. Each segment is assigned a slip deficit accumulation rate and the total surface deformation is calculated by summing the contribution from each segment. The convergence angle between the Juan de Fuca plate and the Cascadia forearc is unknown and is the 16^th free parameter in the dislocation model. In addition to determining the 16 dislocation parameters, there are four regional strain rates ε̇_EE, ε̇_EN, ε̇_NN, and ε̇_UE (regional tilt - positive seaward), and three block rotation parameters λ_P (pole latitude), φ_P (pole longitude), and ω (angular rotation rate). The 23 model parameters are determined simultaneously by using a constrained, weighted, non-linear least square technique. The slip deficit accumulation rate along the plate interface is required to be non-negative and vary smoothly. Confidence limits for each parameter are determined using a bootstrap method. The best model indicates that a slip deficit is accumulating along the plate interface to about 70km downdip from the trench at a maximum of 20mm/yr. The azimuth of slip deficit accumulation is 76.3% ± 2.7 °E which is consistent with the convergence direction of the Juan de Fuca plate and the rotating forearc. The model also indicates that the forearc is rotating relative to stable North America ω =-1.2± 0.1 °/yr about a pole λ_P=43.04 ± 0.3 °N, φ_P=119.18± 0.3°W and deforming internally with ε̇_EE=-3.0± 3.0 nstrain/yr, ε̇_EN =-7.7 ± 2.1 nstrain/yr, and ε̇_NN=-3.1± 0.6 nstrain/yr. The region also appears to be tilting landward, ε̇_UE=-2.7±0.9 nradians/yr.
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7299 General or miscellaneous
DE: 8104 Continental margins: convergent
SC: Seismology [S]
MN: Fall Meeting 2005