The geodynamic evolution of the western part of the Sunda arc is controlled by the change from frontal subduction of the Indo-Australian plate along Java to oblique subduction along Sumatra. This obliquity gives rise to the Sumatra fault zone that links the accretionary zone of the Andaman Sea to the Sunda Strait. Previous studies have shown a decrease of displacement rate of the movement along the fault zone from north to south. Consequently, it has been proposed that the area between the subduction zone and the fault zone—i.e., the Sumatra sliver platelet—was deformed. An oceanographic cruise on the Indonesian ship R/V Baruna Jaya III was designed to study this area. Seismic reflection data reveal the existence of a major strike-slip fault, parallel to the Sumatra fault zone, that we called the Mentawai fault zone, located in the fore-arc area just east of the Mentawai Islands; it is at least 600 km long. Thus, the Sumatra sliver plate appears to be composed of several strips that move toward the northwest to accommodate the oblique subduction.