Solid Earth Constraining fault interpretation through tomographic velocity gradients: application to northern Cascadia

Abstract. Spatial gradients of tomographic velocities are seldom used in interpretation of subsurface fault structures. This study shows that spatial velocity gradients can be used effectively in identifying subsurface discontinuities in the horizontal and vertical directions. Three-dimensional ve-locity models constructed through tomographic inversion of active source and/or earthquake traveltime data are gener-ally built from an initial 1-D velocity model that varies only with depth. Regularized tomographic inversion algorithms impose constraints on the roughness of the model that help to stabilize the inversion process. Final velocity models ob-tained from regularized tomographic inversions have smooth three-dimensional structures that are required by the data. Fi-nal velocity models are usually analyzed and interpreted ei-ther as a perturbation velocity model or as an absolute ve-locity model. Compared to perturbation velocity model, ab-solute velocity models have an advantage of providing con-straints on lithology. Both velocity models lack the ability to provide sharp constraints on subsurface faults. An inter-pretational approach utilizing spatial velocity gradients ap-plied to northern Cascadia shows that subsurface faults that are not clearly interpretable from velocity model plots can be identified by sharp contrasts in velocity gradient plots. This interpretation resulted in inferring the locations of the Tacoma, Seattle, Southern Whidbey Island, and Darrington Devil’s Mountain faults much more clearly. The Coast Range Boundary fault, previously hypothesized on the basis of sed-imentological and tectonic observations, is inferred clearly from the gradient plots. Many of the fault locations imaged from gradient data correlate with earthquake hypocenters, in-dicating their seismogenic nature.