Geophysical image of the hydrothermal system of Merapi volcano

• Published in: Journal of volcanology and geothermal research Vol. 329; pp. 30 - 40
• Main Authors: Byrdina, S., Friedel, S., Vandemeulebrouck, J., Budi-Santoso, A., Suhari, Suryanto, W., Rizal, M.H., Winata, E., Kusdaryanto
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.01.2017
• Subjects: ERT; Hydrothermal system; Merapi; Merapi; ERT; Hydrothermal system
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2016.11.011

We present an image of the hydrothermal system of Merapi volcano based on results from electrical resistivity tomography (ERT), self-potential, and CO2 flux mappings. The ERT models identify two distinct low-resistivity bodies interpreted as two parts of a probably interconnected hydrothermal system: at the base of the south flank and in the summit area. In the summit area, a sharp resistivity contrast at ancient crater rim Pasar-Bubar separates a conductive hydrothermal system (20–50Ωm) from the resistive andesite lava flows and pyroclastic deposits (2000–50,000Ωm). The existence of preferential fluid circulation along this ancient crater rim is also evidenced by self-potential data. The significative diffuse CO2 degassing (with a median value of 400gm−2d−1) is observed in a narrow vicinity of the active crater rim and close to the ancient rim of Pasar-Bubar. The total CO2 degassing across the accessible summital area with a surface of 1.4 ⋅ 105m2 is around 20td−1. Before the 2010 eruption, Toutain et al. (2009) estimated a higher value of the total diffuse degassing from the summit area (about 200–230td−1). This drop in the diffuse degassing from the summit area can be related to the decrease in the magmatic activity, to the change of the summit morphology, to the approximations used by Toutain et al. (2009), or, more likely, to a combination of these factors. On the south flank of Merapi, the resistivity model shows spectacular stratification. While surficial recent andesite lava flows are characterized by resistivity exceeding 100,000Ωm, resistivity as low as 10Ωm has been encountered at a depth of 200m at the base of the south flank and was interpreted as a presence of the hydrothermal system. No evidence of the hydrothermal system is found on the basis of the north flank at the same depth. This asymmetry might be caused by the asymmetry of the heat supply source of Merapi whose activity is moving south or/and to the asymmetry in topography caused by the presence of Merbabu volcano in the north. On the basis of our results we suggest that stratified pyroclastic deposits on the south flank of Merapi screen and separate the flow of hydrothermal fluids with the gaseous part rising through the crater rims, while the liquid part is flowing downwards to the base of the edifice. •Electric resistivity tomography was carried out along the 10 km long north-south profile on Merapi volcano.•Two low-resistivity hydrothermal bodies are identified, one below the summit and the second below the south flank.•Deep resistivity is an order of magnitude higher below the north than below the south flank.•South flank of Merapi shows stratified layers of extremely resistive lava flows followed by conductive water saturated layers.•Stratified deposits separate the hydrothermal fluids: gas rises to the crater, liquid flows to the base of the edifice.