Shallow Ground Temperature Anomaly and Thermal Structure of Merapi Volcano, Central Java, Indonesia

Merapi volcano, in the central part of Java, is regarded as the most active and most dangerous volcano in Indonesia Understanding the thermal structure of the volcano is a matter of great interest. We gathered shallow ground temperature measurements on the flanks of the volcano. A numerical model was proposed in order to understand temperature and pressure distributions during a period of continuous eruptive activity. We simulated numerically the temperature and pressure distributions of Merapi volcano. Firstly, we calculated the steady state fluid flow to determine the background temperature and pressure distributions of Merapi volcano. For the steady state model, we assumed the absence of magma. The dominant water flow pattern within the volcano is downwards. The next step was to model the internal temperatures caused by the intrusion of magma into the conduit and the deeper magma chamber. Based on geochemical data, the intrusive magma temperature is estimated at 900°C and the magma can be distinguished into two parts. The shallower thin conduit is located between 1.5 and 8.7 km below the summit. The deeper magma chamber is located 8.7 km below the summit. The 300°C to 400°C calculated isotherm coincides well with the boundary between brittle and ductile zones obtained from the hypocenter distribution. The model also supports the possibility of a hydrothermal system and ground temperature anomaly in the shallow part of the volcano.