Bathymetry of the Indonesian Sunda margin-relating morphological features of the upper plate slopes...

Abstract. Earthquake history shows that the Sunda subductionzone of the Indonesian margin produces great earthquakesoffshore Sumatra, whereas earthquakes of comparablemagnitude are lacking offshore Java and the Lesser Sundaislands. Morphological structures in multibeam bathymetricdata across the forearc relate with the extent of the seismogeniczone. Its updip limit corresponds to the slope break,most distinct off Java and Lesser Sunda islands, where wefind coincident narrow, uniform, continuous outer arc ridges.Their landward termination and a shallow upper plate mantlemark the downdip limit of the seismogenic zone. In contrastthe outer arc ridges off Sumatra are wider and partly elevatedabove sea level forming the forearc islands. The downdiplimit of the seismogenic zone coincides with a deeper upperplate mantle. Sunda Strait marks a transition zone betweenthe Sumatra and Java margins. We find the differences alongthe Sunda margin, especially the wider extent of the seismogeniczone off Sumatra, producing larger earthquakes, toresult from the interaction of different age and subduction directionof the oceanic plate. We attribute a major role to thesediment income and continental/oceanic upper plate natureof Sumatra/Java influencing the composition and deformationstyle along the forearc and subduction fault.