Subsurface structure of Bali Island inferred from magnetic and gravity modeling: new insights into volcanic activity and migration of volcanic centers

Bali island is part of the Sunda volcanic arc with an active volcano at its center. The Buyan-Bratan caldera, the Batur volcano, and the Agung volcano are active volcanoes aligned from the west to the east of the island of Bali. The activity and age of volcanoes progress eastward. Therefore, the physical properties and conditions of each body of magma beneath the volcanoes on the island of Bali requires investigation. Ground magnetic measurements were carried out throughout the island and integrated with the Bouguer anomaly map. Hand samples were taken that represented each lithology of Bali Island. Furthermore, magnetic susceptibility and density measurements were carried out, which were used as initial data for generating magnetic and gravity models. Apart from that, geological maps and previous geological and geophysical research in the Bali Island area were also used as supporting data to create magnetic and gravity models. Magnetic and gravity modeling shows a carbonate rock formation in Bali Island's southern volcanic area, surrounded by a graben up to 6 km deep and covered by volcanic deposits. Furthermore, magnetic and gravity models indicate an intrusive rock between Agung and Batur volcanoes as well as shallow and deep magma chambers beneath the Buyan-Bratan Caldera, Agung, and Batur volcanoes. The magnetic susceptibility and density differences from the magnetic and gravity models in this study are one way to trace the eastward shift in volcanic activity on Bali Island. These findings will contribute to better understanding the volcanic processes and the migration of volcanic centers, which can help volcanic and earthquake hazard mitigation.

Graphical abstract