Six refraction lines were shot parallel to structure, from the Sunda Trench axis to the west coast of Sumatra, near Nias Island. Seaward of the Nias Island ridge, 2.4--3.8 km of low-velocity sediment (2.0--3.6 km/s) is underlain by a wedge of 4.7- to 4.9--km/s material, probably dewatered sediment and oceanic basement. The wedge thickens from 1.2 km beneath the trench axis to 13.5 km immediately seaward of the ridge. layer 3, 6.6--7.5 km/s and 3.7--23 km thick, dips 5¿ landward beneath this wedge. The large variation in layer 3 thickness may be evidence that imbricate thrust faulting, responsible for the thickening which forms the 4.7-to 4.9-km/s wedge, also extends downward into layer 3. The forearc basin, landward of Nias Island, has 3.0 km of low-velocity sediment overlying 19 km of 6.5-km/s material and 8.1-km/s Moho. Near the coast of Sumatra, layers of 2.1, 4.3, 5.3, and 6.0 km/s, whose thicknesses total 8.1 km, are underlain by a large, undetermined thickness of 6.8-km/s material. This 6.5- to 6.8-km/s layer may be either lower continental (granitic) crust or thickened oceanic layer 3.