Self-potential and ground temperature at Merapi volcano prior to its crisis in the rainy season of 2000–2001

Rainwater infiltration into the active lava dome of Merapi volcano (Central Java) has often been suggested as a triggering mechanism for partial dome collapse and subsequent eruptive behavior. However, the phenomenon has not been systematically investigated. We use a multi-parametric approach to study the interaction of the hydrologic and magmatic system near the lava dome prior to its crisis in early 2001. We describe the hardware concept for a continuous monitoring system for self-potentials (U, SP) and ground temperature T installed in August 2000 at the solfatara Woro only 150 m from the active lava dome. Relating the data from this station to measurements of rainfall, air pressure, solfatara temperature and seismicity, several effects can be distinguished: (a) A long-term correlation is observed between electrode temperature difference and SP with estimated coupling coefficients of 2.1±0.5 mV/°C. (b) After individual rainfall events, U–T signatures show a typical pattern of positive charge accumulation at the hot electrodes near the fumarolic vents. (c) An exceptional anti-correlation between U and ΔT is observed in January 2001, during a period of rapid dome growth prior to the collapse and eruptive crisis. (d) Wavelet analysis of SP reveals the sporadic occurrence of periodic fluctuations with a clear 12-h (S2) component. Because of their amplitude of up to 100 mV/km and their periodicity (S2 but not 24 h or lunar 12 h 25 min), these fluctuations are unlikely to be caused by natural magnetic field variation or by earth tides. Instead, they do seem correlated but not in phase with atmospheric pressure changes. We discuss effects (a–d) in the framework of source mechanisms related to water–lava interactions.