Subsurface Evidence of the Ancient Muria Strait and Preliminary Geohazard Implications from Airborne Gravity and EMAG2-v3 Data

Abstract

The Muria Peninsula in northern Central Java records interacting volcanic, tectonic, coastal, and fluvial processes, but the subsurface geometry of the former marine passage south of the Muria volcanic complex remains uncertain. This study uses airborne gravity as the primary quantitative dataset for three-dimensional density modelling, while EMAG2-v3 is used as complementary regional magnetic context. Airborne free-air gravity data were processed to the Complete Bouguer Anomaly (CBA), separated into regional and residual components, and inverted using SimPEG. The density inversion is expressed as density contrast relative to a reference density of 2.0 g/cm³ and converted to absolute density for geological interpretation. A laterally continuous low-density corridor with interpreted absolute densities of approximately 1.4–1.9 g/cm³ occurs south of Mount Muria and is interpreted as geophysical evidence consistent with sedimentary infill of the Ancient Muria Strait. Profiles L4 and L5 indicate an interpreted corridor width of approximately 15–30 km, with uncertainty related to flight-line spacing, regularisation, and the selected density threshold. EMAG2-v3 shows a broadly weak magnetic response over parts of the same corridor; however, the magnetic result is treated as an effective magnetic susceptibility model under an induced-magnetization assumption and is not used as a quantitative constraint on the gravity inversion. The gravity inversion achieves RMSE ≈ 1.07 mGal and R² = 0.994, which indicate data fit but do not demonstrate geological uniqueness. The inferred soft-sediment corridor has preliminary geohazard implications and may warrant targeted geotechnical investigation rather than being interpreted as a direct hazard assessment.