The 2009 Samoa tsunami resulted in severe damage and inundation at Tutuila, American Samoa. The disparity of the impact and the varying accounts of the tsunami along the coast remain one of the most intriguing aspects of the event. We utilize a dispersive wave model to reconstruct the tsunami from the earthquake source for understanding of the wave dynamics around Tutuila. After validation with water-level measurements, a Fast Fourier Transform of the computed surface elevation reveals coupled resonance oscillations between 3 and 18 min period over the insular slope and shelf as well as the fringing reefs. The resonance, which focuses energy according to shelf and embayment configurations, provides an explanation of the runup data and eyewitness accounts and identifies the coastal communities prone to tsunami hazards.