Imaging the Seattle fault zone with high-resolution seismic tomography

The Seattle fault, which trends east-west through the greater Seattle metro politan area, is a thrust fault that, around 1100 years ago, produced a maj or earthquake believed to have had a magnitude greater than 7. We present t he first high resolution image of the shallow P wave velocity variation acr oss the fault zone obtained by tomographic inversion of first arrivals reco rded on a seismic reflection profile shot through Puget Sound adjacent to S eattle. The velocity image shows that above 500 in depth the fault zone ext ending beneath Seattle comprises three distinct fault splays, the northernm ost of which dips to the south at around 60 degrees. The degree of uplift o f Tertiary rocks within the fault zone suggests that the slip-rate along th e northernmost splay during the Quaternary is 0.5 mm a(-1), which is twice the average slip-rate of the Seattle fault over the last 40 Ma.