Seismic refraction studies of the Sunda Trench and Forearc Basin

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Abstract
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.