Inner structure of the Krakatau volcanic complex (Indonesia) from gravity and bathymetry data

On 27 August 1883, the Krakatau volcanic complex (Indonesia) was the site of one of the most destructive historical eruptions. Most of the volcano was destroyed and a new caldera also formed during this catastrophic event. Since the date of the eruption, many geological studies of the superficial structures and eruption products have been carried out. A debate on the scenario of the eruption and the way the volcano collapsed has developed and still is unresolved. In order to assess the inner structure of the volcanic complex, we carried out a detailed land and marine geo-physical survey in the summer 1990. In this paper, bathymetry and gravity data collected during the survey as well as literature data are compiled and analysed. Bathymetric data show that the caldera is characterized by a flat seabottom at 240 m below sea level and by steep linear walls suggesting that the caldera collapse has been controlled by pre-existing features. Moreover, the build-up of the young active volcano, Anak Krakatau, on the very edge of the caldera could lead to mechanically unstable conditions which must be considered for hazard mitigation. The Bouguer anomaly of the volcanic complex is characteristic of volcanoes with an explosive behaviour. 3-D gravity modelling reveals the previously unknown geometry of the dense substratum of the proto-Krakatau and evidences the presence of a collapsed structure beneath the caldera filled up with low-density material. Finally, we point out a major weakness zone, oriented N150 ° on a line passing through the old and recent vents of the volcano. This zone could have guided both the development of the volcanic activity and the emplacement of the 1883 caldera. Furthermore, this weakness zone passing through the summit line of the pre-1883 Krakatau volcano has been introduced as a significant disruption surface of the volcanic edifice in the updated scenario of the 1883 eruption that we propose.