A major earthquake on the Seattle fault, Washington, ca. A.D. 900–930 was first inferred from uplifted shorelines and tsunami deposits. Despite follow-up geophysical and geological investigations, the rupture parameters of the earthquake and the geometry of the fault are uncertain. Here we estimate the fault geometry, slip direction, and magnitude of the earthquake by modeling shoreline elevation change. The best fitting model geometry is a reverse fault with a shallow roof ramp consisting of at least two back thrusts. The best fitting rupture is a SW-NE oblique reverse slip with horizontal shortening of 15 m, rupture depth of 12.5 km, and magnitude Mw = 7.5.