FINITE-ELEMENT STUDY OF UPLIFT AND STRAIN ACROSS VANCOUVER-ISLAND

Geological evidence for sudden coastal subsidence along the west coast of southern Vancouver Island points to the occurrence of great prehis torical subduction earthquakes. Contemporary uplift and crustal shorte ning patterns in southern Vancouver Island appear to indicate that the subduction megathrust fault is currently locked. To understand better the dynamics of the observed surface deformation, we develop a finite element model of earthquake cycles for the northern Cascadia subducti on zone across southern Vancouver Island, using a linear viscoelastic theology. The model consists of the continental and oceanic lithospher es, the asthenospheric mantle with a viscosity of 5 x 10(19) Pa.s, and a low-viscosity (10(18) Pa.s) mantle wedge between the subducted ocea nic plate and the overlying continental plate. The shallow geometry of the subducted Juan de Fuca plate is well defined by the results of va rious geophysical surveys, and the deep geometry is constrained by the results of seismic tomography. The model megathrust fault has a stick -slip zone near the surface, a viscoelastically weakly coupled zone (v iscosity 7 x 10(17) Pa.s) at depth, and a narrow free-slip zone in bet ween. Earthquakes are allowed to occur every 500 years. Varying the re currence time does not greatly affect the surface deformation in the l ater part of the interseismic period. Experiments varying the width of the stick-slip zone lead to the conclusion that a width of about 70 k m satisfies both the observed coseismic coastal subsidence and the con temporary surface deformation pattern. The results of a simple elastic dislocation model for thrust earthquakes that had been previously app lied to the region are compared with the solutions of the viscoelastic model. Despite its simplicity, the elastic model approximates well th e surface deformation of the viscoelastic model in the second half of the interseismic period, although it predicts a slightly narrower stic k-slip zone of the fault. The present viscoelastic model is limited pr incipally by the two-dimensional approach, the assumptions of purely s tick-slip behaviour of the thrust fault, and the uncertainties in rock theology.