| Gravity Analysis of a Subsurface Fault Controlling the Sector Collapse of Galunggung Caldera, Indonesia |
| Muhammad Naufaldi, Izzuki Hamida, Ilham Arisbaya, Yayat Sudrajat, Sungkono Sungkono, M. Altin Masinai, Hendra Grandis |
| Abstract Galunggung volcano (7°15′S, 108°3′E) in West Java, Indonesia, is an active stratovolcano with a significant history of eruptions and widespread impacts on the surrounding area. One major eruption occurred around 4,200 years ago, causing a sector collapse in the southeastern part of the mountain and forming a caldera that shaped the landscape known as the Thousand Hills of Tasikmalaya. Previous studies explained the cause of this collapse as triggered by a massive eruption or by the steep-slope control to the southeast. However, both suggest a tectonic role that may have contributed to the flank collapse. The distribution of hot springs around Galunggung, which forms a lineament toward the caldera, may provide a clue regarding the factors controlling the sector collapse event. We analyzed 117 legacy gravity measurements collected in 2000 and reprocessed them using advanced techniques to examine the subsurface structure and investigate the possibility of a subsurface weak zone. Bouguer anomaly separation using a Butterworth Band-pass Filter and derivative analysis (FHD, FVD, TDX) revealed a linear NW-SE trending low-gravity anomaly (32.5–39.4 mGal) extending ~6 km southeast of Galunggung, indicating the presence of a subsurface fault. This interpretation is supported by the alignment of the distribution of geothermal manifestations and the orientation of the caldera’s morphology, which opens in the same direction. This NW-SE inherited fault structure is interpreted to have played a dual role in the volcano dynamics, serving both as a pathway for the migration of hot fluid, which may decrease rock strength, and as a structural weakness intruded by magma. Thus, we propose that this pre-existing fault was a controlling factor in the collapse of Galunggung through a fault-parallel breaching mechanism. These findings have important implications for volcanic hazard assessment in densely populated areas around Galunggung. |
Tektonesiana 