Regional bouguer anomaly gravity data: 3-D modelling of subsurface structures of the flores and timor earthquake risk area

Regional anomaly gravity data are very useful for determining deep subsurface structures due correspond deep surface anomalies. This research aims at analyzing qualitatively to map the gravity anomaly of the island of Flores, modelling shallow surfaces, and interpret surface structures based on the gravity anomalies. In this research, the gravity method used was the secondary gravity data from the Bureau Gravimetric International (BGI) that would be applied using Upward Continuation techniques to interpret deep surfaces for 3D modelling of earthquake risk areas on the islands of Flores and Timor. The modelling was performed by an inversion technique applying the method of singular value decomposition (SVD) and Occam inversion. The result showed that the subsurface structure of the volcano area of the Flores island consists of sandstone, breccia, and andesite with density values of 2.42 g/cm³ - 2.62 g/cm³ and basaltic and lava with density values of 2.65 g/cm³ - 3.24 g/cm³. The most dominating rock in the study area was basaltic rocks with the average density of 2.73 g/cm³. Based on 3D models of regional Bouguer gravity anomalies, the Flores back arc fault was estimated at a depth of 30 km, the thickness of the earth's crust in the Flores back arc fault zone was estimated to be up to 30 km while the thickness of the earth's crust in the fore arc was estimated to be more than 60 km. The existence of magma chamber in the volcanic arc of Flores island was estimated at depth of 30 km in accordance with to the seismicity map Flores and Timor areas.