Sunda Trench an underwater ridge, and much of the trench is at least partially filled with sediments. Mappings after the 2004 Indian Ocean earthquake of the plate boundary showed resemblance to suspension bridge cables, with peaks and sags, indicative of asperity and locked faults, instead of the traditional wedge shape expected. Some of the earliest exploration of the Trench occurred in the late 1950s when Robert Fisher, Research Geologist at the Scripps Institution of Oceanography, investigated the trench as part of a worldwide scientific field exploration of the world's ocean floor and sub-oceanic crustal-structure. Bomb-sounding, echo-train analysis and manometer were some of
Sumatra Trench to 350 km. The width at Sumatra trench is controlled by sediment input and accretion rate and history. One distinguishing feature of the Sumatra subduction zone is that it has a relatively 120–140 km accretionary prism and a deep forearc basin (so does the rest of the whole Java subduction zone). However it does vary along the trench. The prism is broad and has a relatively shallow surface slope in the northern part, becomes steep and narrow at the central part, then becomes steep and narrow in the southern area. The inner part of the prism forms a NW-SE arc