Offshore extension of the Great Sumatra Fault revealed by seismic, bathymetric and seafloor imaging

The northwestern Sunda Arc is cut by two giant dextral shear zones, which accommodate the oblique subduction of the Indo-Australian Plate against Eurasia along this portion of the Sunda Arc. These are the Sumatra Fault Zone on Sumatra Island and the Mentawai Fault Zone off west Sumatra. The strike-slip shear zones bear a high earthquake risk potential. Several Indonesian-Japanese-German marine expeditions focused on the tectonics and the evolution of the Sumatra and Java forearc region. Now a dense grid of multi- and single-channel reflection seismic profiles, high resolution bathymetry and seafloor direct observations using submersible SHINKAI 6500 allow tracing the Sumatra and Mentawai Fault Zones south off west Java. The geometry of transtensional basins, transpressional horst structures and elongated ridges and valleys clearly shows the dextral sense of motion and the splay character of the fault zones in the study area. Multi-channel reflection profiles image the deep structure of the major strike slip faults which originate from reactivated thrust planes within the Java accretionary wedge. During SHINKAI 6500 dives one master fault could be followed. Recent offset along the fault is documented on sea floor images by young unweathered fault scarps.