We have produced a large set of broadband (0-10 Hz) synthetic seismograms for Mw 9.0 earthquakes on the Cascadia megathrust by combining synthetic seismograms derived from 3D finite-difference simulations ( 1 Hz) with finite-source, stochastic synthetics ( 1 Hz). We used a compound rupture model consisting of high stress drop M8 sub-events superimposed on large, shallower slip with long slip duration, informed by observations of the M9.0 Tohoku, Japan and M8.8 Maule, Chile earthquakes. Fifty 3D simulations were run, considering a variety of rupture parameters, to determine the range of expected ground motions. For sites not in sedimentary basins, the spectral accelerations of the synthetics are similar to the BC Hydro ground-motion prediction equations for periods of 0.1-6 s, but exceed them at periods greater than 6 s. Response spectra from the synthetics show large basin amplification factors of 2-5 at periods of 1-10 s for sites in the Seattle and Tacoma sedimentary basins.