SEISMIC COUPLING ALONG THE CASCADIA SUBDUCTION ZONE DETERMINED BY SIMULTANEOUS INVERSION OF HORIZONTAL AND VERTICAL SURFACE DEFORMATION DATA

In most models of Cascadia subduction, lithology and heat flow models constrain the downdip extent of coupling between the subducting and overriding plates. Recent seismological studies suggest that some of the previous assumptions made regarding lithology may be unjustified. Dislocation models suggest that active surface deformation can be the result of deep coupling in addition to the shallow region that is indicated by currently accepted models. The modeling also suggests that the slip deficit associated with coupling is not accumulating uniformly along the strike of the subduction zone.

A two-dimensional inverse modeling method was developed to simultaneously invert vertical and horizontal deformation data to determine the rate at which slip-deficit is accumulating along the subduction interface. The model uses a smoothed, damped, non negative least squares technique. A standard dislocation model is used where the subduction interface is divided into small fault segments and the amount of slip deficit on each segment is determined by the inversion. In this study, horizontal surface deformation data, derived from GPS campaigns, are combined with surface uplift rates determined from historic leveling surveys.

The data from two nearly east-west profiles are inverted. A profile running through the southern Puget Sound region indicates two distinct regions of slip deficit accumulation. One shallow region extends eastward from the toe of the wedge to beneath the coastline. The second region is located further down dip beneath the Puget Sound. Data from another east-west profile indicate slip-deficit is only accumulating in a single shallow region at the latitude of the Columbia River.