Geophysical image of the hydrothermal system of Merapi volcano

2017

Article référencé dans le Web of Science WOS:000392553700003

Byrdina Svetlana, Friedel S., Vandemeulebrouck J., Budi-Santoso A., Suhari, Suryanto W., Rizal M. H., Winata E., Kudaryanto

Journal of Volcanology and Geothermal Research, 2017, 329, p. 30-40 ISSN 0377-0273

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 (20-50 ohm m) from the resistive andesite lava flows and pyroclastic deposits (2000-50,000.0 m). 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 400 g m(-2) d(-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 1.4.10(5) m(2) is around 20 t d(-1). Before the 2010 eruption, Toutain et al. (2009) estimated a higher value of the total diffuse degassing from the summit area (about 200-230 t d(-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 100,000 ohm m, resistivity as low as 10 ohm m has been encountered at a depth of 200 m 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.

Hydrologie [062] ; Géologie et formations superficielles [064] ; Géophysique interne [066]

INDONESIE ; JAVA ; MERAPI VOLCAN

Fonds IRD [F B010068903]

fdi:010068903