Deep sedimentary basins amplify long-period shaking from seismic waves, increasing the seismic hazard for cities within such basins. We perform 3-D simulations of point source earthquakes distributed around the Seattle and Tacoma basins in Washington State, to examine the dependence of basin amplification on source azimuth, depth, and earthquake type. For periods between 1-10 s, the pattern of amplification is spatially heterogeneous and differs considerably with the source-to-site azimuth. For close-in earthquakes, the greatest basin amplification occurs towards the far side of the basin and ground motions from crustal earthquakes experience greater amplification than those from more vertically-incident, deeper intraplate earthquakes. Love and Rayleigh waves form similar spatial patterns for a given source location, although the magnitude of amplification varies. The source dependence of basin amplification is an important factor for seismic hazard assessment, both in the Seattle and Tacoma basins, and by extension for deep sedimentary basins worldwide.