Geological structure of the offshore Sumatra forearc region estimated from high-resolution MCS reflection survey

•High-resolution MCS reflection survey is conducted offshore northwestern Sumatra.•Detailed fault distributions and shallow geological structure can be clarified.•Two kinds of different vergence systems are characterized in the Sumatra forearc.•Recent deformation is recognized in the piggyback basin in the forearc high.•Many of recent deformations are distributed along the Middle Thrust.

To investigate detailed fault distributions and shallow geological structure offshore northwestern Sumatra, we obtained high-resolution Multi-Channel Seismic (MCS) reflection data around the Sunda Trench, trench slope, and forearc high regions offshore northwestern Sumatra. In general, trench-parallel anticlinal ridges are distributed from trench slope region to forearc high region. Two kinds of different vergence systems are characterized in the Sumatra forearc region; landward vergence is dominant in the lower trench slope region, and seaward vergence is dominant in the forearc high region. Moreover, piggyback or slope basins are recognized between anticlinal ridges. Deformation in the uppermost part of these basins, that is referred to 'recent' deformation in this paper, can be identified not only along major thrusts but also between major thrusts and the lower trench slope, suggesting these are related to recently active faulting. Several but the largest number of such deformation are distributed along a major thrust located in the middle of the forearc high region, whereas few are done along other major thrusts.