Upper-mantle structure of the Cascadia subduction zone from non-linear teleseismic travel-time inversion

Abstract

The tectonic evolution of the Juan de Fuca plate system has undergone dramatic changes over the past several million years as evidenced by seafloor geomagnetic data. If there are large geodynamic forces active in the subduction of oceanic lithosphere, corresponding changes in the structure of the Cascadia subduction zone should also be dramatic. In light of the seismic hazard normally presented by active subduction zones and the Pacific Northwest population centers potentially affected by such a hazard, it is important that we understand this subduction zone structure in order to properly evaluate the seismic potential of the region. We have approximated a non-linear inversion for variations in seismic wave speed over the upper-mantle portion of the Cascadia subduction zone by alternately performing linear travel-time inversions, via an efficient conjugate gradient method, and three-dimensional ray tracing. We employ regularization through minimizing the Laplacian of the final solution. We parameterize the three-dimensional model at over 10,000 knots with velocities between knots represented by spline interpolation. The culled data set consists of approximately 10,000 relative arrival times of teleseismic P and PKP waves recorded from 1980 to 1990 at 146 station locations of a short-period vertical-component seismic network in Washington and Northern Oregon. We obtain accurate relative arrival-time data (standard errors $\approx$0.01 sec) and associated data covariance estimates via the use of a new multi-channel cross-correlation technique. The most prominent and robust characteristic of our models is a 3-4% fast, planar feature that dips steeply to the east (at $\approx$60$\sp\circ$) with a width of approximately 100 km. We infer this to be the seismic manifestation of a thermal and compositional anomaly associated with the subducting Juan de Fuca oceanic plate. At shallow depths ($\approx$80 km) this feature is consistent with the projections of models of shallow slab structure. The high velocity zone is located at a depth of 100-120 km beneath the Cascade volcanos, consistent with other subduction zones. Under central Washington the slab extends to depths of 400 km or more, while there is an apparent lack of deep slab material beneath southern Washington and northern Oregon. The latter, when taken together with the tectonic history of the region and other geophysical observations, is consistent with a deep slab that has torn away from the shallow portion of the slab.