Subsurface Modeling Using Gravity Data in Southern Yogyakarta and Its Implications for The May 26th, 2006 Earthquake

Abstract

The Yogyakarta region is one of the most seismically vulnerable areas in Indonesia, so detailed subsurface fault characterization is crucial for effective disaster mitigation strategies. Previous studies have recognized the Opak Fault as the main structure controlling seismicity in this area, but the geometry and existence of other fault systems remain less constrained. This study aims to delineate the subsurface density variations associated with fault zones and rock deformation. This study develops a two-dimensional gravity inversion model to refine the subsurface configuration of faults in southern Yogyakarta. Gravity data were processed through standard corrections and anomaly separation to generate a complete Bouguer anomaly and residual anomalies. The results show that residual anomalies (-60 to 100 mgal) successfully delineate lithological variations, where older volcanic units (Semilir, Nglanggeran, and Sambipitu Formations) produce positive density contrasts (0.4-0.85 g/cm³), while younger Merapi deposits display negative contrasts (-0.5 to -0.05 g/cm³). Importantly, the 2D model highlights not only the Opak Fault but also the Ngalang Fault to the east, spatially coinciding with the 26th May 2006 earthquake epicentre. This finding provides new evidence of the structural complexity in southern Yogyakarta and emphasizes the role of the Ngalang Fault in regional seismicity, offering an updated basis for seismic hazard assessment in the area.