Relative tectonic activity assessment of the northern sumatran fault from geomorphological and morphotectonic analyses

The Northern Sumatran Fault is a part of a trench-parallel strike-slip fault situated in oblique subduction setting. This study aims to assess the relative tectonic activity using geomorphic indices and to interpret the basin-forming mechanisms from geomorphic expressions in order to construct the schematic evolution of the Northern Sumatran Fault based on the development of transverse drainage basins and streams. This study utilized the 8.25-m digital elevation model released by the Geospatial Information Agency, the Republic of Indonesia, namely the DEMNAS. This study applied six geomorphic indices for assessing the relative tectonic activity: mountain-front sinuosity (Smf; average 1.14—1.94), valley width-to-height ratio (Vf; average 0.58—1.93), stream length-gradient index (SL; average 93.81—599.70), index of basin shape (Bs; average 1.66—3.16). asymmetric factor (AF; average 7.45—19.93), and hypsometric integral (HI; average 0.41—0.59). The index of relative tectonic activity (Iat) derived from these indices shows alternating moderate and high tectonic activities along the Northern Sumatran Fault, with northward decrease in its northern section. The fault section with high and very high tectonic activities coincides with greater occurrences of earthquakes, while the northernmost fault section with moderate tectonic activity lacks earthquakes. Furthermore, the geomorphic expressions were studied to interpret the fault configuration, stream deflection, and the delineation of landforms. The development of transverse drainage basin was examined with drainage basin relief ratio (Rh; average 0.08—0.30) and drainage basin volume-to-area ratio (Rva; average 12.18—101.66). For the stream profile analysis, this study applied normalized stream profile, qualitative interpretation of profile shape, stream concavity index (SCI; average -0.02—0.13), and stream gradient (average 0.05—0.25). This study interprets that the Northern Sumatran Fault has six basin-forming mechanisms: fault-angle basin, transpressional basin, fault-bend basin, stepover basin, fault-junction basin, and pull-apart basin. The direction of fault propagation was interpreted based on the relative timing of basin formation. The Northern Sumatran Fault is interpreted to propagate towards its center. The interpreted direction of fault propagation coincides with the direction of increasing numbers of earthquakes.