Internal structure of the Merapi summit from self-potential measurements

Recent activity at Merapi volcano occurred mainly inside the avalanche caldera related to a huge, prehistoric Mt. St-Helens-like event (Berthommier, 1990). Since 1786, the activity has been mainly related to the growth and collapse of domes, all contained within the large crater named Pasarbubar. Some events (1822-1823, 1872 and 1930-1931) were more powerful than the average event and formed wide craters. The relative importance of the different craters and their vent locations are not well known, because of thick deposits, which partially hide these structures. We applied the self-potential (SP) method in 1990-1991 in order to improve our knowledge of these structures, in the area between the crater Pasabubar and the present dome. SP measurements allow us to partially identify the geometry of the different craters and thickness variations of the deposits inside the craters. The SP profile show two main negative anomalies, interpreted to represent the crater Pasarbubar, and the 1822 crater. The 1872 crater is only partially recognizable on the profile. From the SP anomalies, it is possible to estimate the vadose-zone thickness, but not the full thickness of the crater fill material. North from the summit, one traverses a positive anomaly. The northern limit of this anomaly, as observed on the profile, is interpreted as the limit of the 1930-1931 crater. This crater is now entirely filled by a dome complex, involving the present and older domes. The high, regular SP values occupying the entire summit region are interpreted to reflect the presence of a convective hydrothermal cell. The risk of an important slide from the wall of these different craters should he considered, as was the case of the prehistoric Mt.-Saint-Helens-type event. (C) 2000 Elsevier Science B.V. All rights reserved.