ALBERT

Description: T13A-2185 Earthquake history shows that the Sunda subduction zone of the Indonesian margin produces great earthquakes offshore Sumatra, whereas earthquakes of comparable magnitude are lacking offshore Java and the Lesser Sunda islands. We use morphological structures in multibeam bathymetric data across the forearc to identify the extent of the seismogenic zone (SZ). The updip limit of the SZ is associated with a distinct slope break at the seawordmost part of the outer arc high off Java and the Lesser Sunda islands. In contrast, the slope break is rather indistinctive off large parts of Sumatra. The inner wedge shows differences along the Indonesian margin. Uniform trench-parallel ridge structures lie off Java and Lesser Sunda islands, whereas non-uniform trench-parallel outer arc high structures consisting of several broad tectonic ridges off Sumatra shape the seaward part of the inner wedge. The landward termination of the inner wedge ridge structure and a shallow upper plate mantle at a depth range of ~15-25 km at Java and the Lesser Sunda islands coincide with the downdip limit of the seismogenic zone. In contrast the outer arc ridges off Sumatra are wider and partly elevated above sea level forming the forearc islands. Here, the downdip limit of the seismogenic zone is situated at depths of ~30-40 km, which coincides predominantly with a deeper upper plate mantle. Sunda Strait marks a transition zone between the Sumatra and Java margins. We find the differences along the Sunda margin, especially the wider extent of the seismogenic zone off Sumatra, producing larger earthquakes, to result from the interaction of different age and subduction direction of the oceanic plate. We attribute a major role to the sediment income and continental/island arc upper plate nature of Sumatra/Java influencing the composition and deformation style along the forearc and subduction fault. Off Sumatra the SZ is up to more than twice as wide as off Java and the Sunda islands, enlarging the unstable regime off Sumatra and thus the risk of sudden stress release in a great earthquake.