Locking State of the Northern Cascadia Megathrust Fault Inferred From Inversion of GPS Velocities

In order to assess the state of interseismic locking of the plate boundary fault between the Juan de Fuca (JDF) plate and the North American plate in the northern Cascadia subduction zone (CSZ), we invert GPS horizontal velocities using a method based on Akaike's Bayesian Information Criterion (ABIC). We model the interseismic locking, represented by back slip, along the 3-D curved upper surface of the subducting JDF plate using a 420 x 210 km source region within a purely elastic half-space. GPS velocities from 37 continuous and campaign stations in Western Canada and 5 continuous stations in northern Washington are inverted to determine the back-slip distribution. The observed velocities are characterized by a northeastward motion and a landward decrease away from the CSZ. The reference station at Penticton has little motion relative to the stable interior of the North America plate. In the model, the back-slip distribution is represented by the superposition of 42 basis functions. The back-slip vectors are estimated together with their errors. Preliminary results show northeastward back-slip of 22 to 30 mm/yr at about 10 km depth, consistent with the complete locking of the shallowest part of the thrust fault. The back slip is less than 20 mm/yr at depths deeper than 20 km, and this depth distribution of back slip is consistent with an effective transition zone used in previous forward dislocation models for this region. Our problem of geodetic inversion is severely ill-posed. Various model constraints and parameterization details will yield somewhat different back-slip distributions that all provide seemingly excellent fit to the GPS data. For example, using a narrower back-slip source region will result in larger back-slip vectors. The choice of a preferred model calls for other independent information such as thermal and paleoseimic constraints.