Subsurface structure of Sumani segment in the Great Sumatran Fault inferred from magnetic and gravity modeling

Abstract The Great Sumatran Fault (GSF) 1900 km long active and segmented strike-slip faults. One of the segments is the Sumani segment in the province of West Sumatra that passes through a relatively densely populated area. Knowing the geometry of subsurface structures in this segment is one of the efforts to identify potential disasters in the future. To achieve this objective, ground magnetic measurements were carried out and the data were combined with Bouguer anomaly map. Magnetic susceptibility and density were also measured on hand samples collected during the survey so that their values could serve as initial parameters for modeling. The modeling and interpretation stages are carried out through joint forward modeling magnetics and gravity. The equivalent depth of the magnetic and gravity models built under the surface is 20 km. The magnetic and gravity modeling identify several intrusive bodies that do not reach the surface, including one beneath Lake Singkarak. Several slivers of Permian metamorphic rocks encased in the basement rocks were also identified. These unexposed intrusive bodies and slivers of encased metamorphic rocks are probably quite common in the GSF. The magnetic and gravity modeling with 2D modeling also infers an 18 km displacement in Lake Singkarak based on displacement of Quaternary colluvium volcanic deposits. Based on the presence of hot spring at the end of Sumani segment, the pull-apart pattern occurred in Lake Singkarak share similarities with that occur in another segment of GSF, namely Tarutung.