Imaging the Seattle Fault Zone with high‐resolution seismic tomography

Abstract

The Seattle fault, which trends east‐west through the greater Seattle metropolitan area, is a thrust fault that, around 1100 years ago, produced a major earthquake believed to have had a magnitude greater than 7. We present the first high resolution image of the shallow P wave velocity variation across the fault zone obtained by tomographic inversion of first arrivals recorded on a seismic reflection profile shot through Puget Sound adjacent to Seattle. The velocity image shows that above 500 m depth the fault zone extending beneath Seattle comprises three distinct fault splays, the northernmost of which dips to the south at around 60°. The degree of uplift of Tertiary rocks within the fault zone suggests that the slip‐rate along the 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.