The Eastern United States has a complex geological history and hosts several seismic active regions. We investigate the subsurface structure beneath the broader eastern United States. To produce reliable images of the subsurface, we simultaneously invert smoothed P-wave receiver functions, Rayleigh-wave phase and group velocity measurements, and Bouguer gravity observations for the 3D shear wave speed. Using surface-wave observations (3-250 s) and spatially smoothed receiver functions, our velocity models are robust, reliable, and rich in detail. The shear-wave velocity models fit all three types of observations well. The resulting velocity model for the eastern U.S. shows thinner crust beneath New England, the east coast, and the Mississippi Embayment. A relatively thicker crust was found beneath the stable North America craton. A relatively slower upper mantle was imaged beneath New England, the east coast, and western Mississippi Embayment. A comparison of crust thickness derived from our model against four recent published models shows first-order consistency. A relatively small upper mantle low-speed region correlates with a published P-waves analysis that has associated the anomaly with a 75 Ma kimberlite volcanic site in Kentucky. We also explored the relationship between the subsurface structure and seismicity in the eastern U.S. We found earthquakes often locate near regions with seismic velocity variations, but not universally. Not all regions of significant subsurface wave speed changes are loci of seismicity. A weak correlation between upper mantle shear velocity and earthquake focal mechanism has been observed.