Geophysical Investigations of Ungaran Volcano, Central Java, Indonesia

Abstract

We have been conducting some kinds of investigations for the hydrothermal system of Ungaran Volcano, Indonesia with Gadjah Mada University since 2004. In 2004 and 2005, we mainly researched the Gedongsongo fumarolic area on the southern mountainside of Ungaran Volcano. We carried out infrared and visual imagery observation, and estimated the heat discharge rate of this area at 1.25 MW with the value of the hot springs from the previous study. The result of SP surveys showed some positive anomalies in the vicinity of the fumarole and hot spring zones and the southernmost lower altitude part. The tripartite seismic observation at the Gedongsongo area in 2005 detected active seismicity that includes 4 swarms for 5 days. By using these results, we constructed a conceptual model of the hydrothermal system for the area, which describes that a part of ascending geothermal fluid from the deeper part of Ungaran Volcano changes into a lateral flow, reaches to the ground surface and forms the Gedongsongo fumarolic area. In parallel with these researches, we reanalysed the gravity data of the Ungaran area measured by the Gadjah Mada University team, and drew a Bouguer anomaly map that illustrates the underground structure of Ungaran Volcano.