Magnetic monitoring at Merapi volcano, Indonesia

Abstract Merapi volcano, located 30 km north of the heavily populated city of Yogjakarta, Java, is one of the most active of the 129 volcanoes in Indonesia. About every 2 years a new phase of activity is observed. Depending on the past activity the unrest gives rise either to an endogenous dome which partly collapses in the southwest direction or to pyroclastic flows which travel as far as 15 km. The 1990–1997 period has involved a plume emission on 30 August 1990, an extrusion on 20 January 1992, and a pyroclastic eruption on 22 November 1994. The intensity of the Earth magnetic field has been measured simultaneously and digitally recorded at four stations since 1990. Two Overhauser magnetometers with resolution of 0.01 nT have been installed in the summit area to strengthen the volcano monitoring. Outstanding magnetic changes appear to correlate with volcanic activity. Three types of volcanomagnetic signals can be identified: long-term trends up to 15 nT with period >10 years; medium-term cyclic variations, at most 3 nT in amplitude and with 1–2 years period; and small events, reaching 1.5 nT, lasting a few months, and associated with any remarkable volcanic activity. Merapi volcano began a new cycle of activity in 1995 leading to a dome growth in July 1996, and accompanied by 27 nuees ardentes in August. The comparison between magnetic data, seismicity, and surface phenomena suggests that some long-term trends of decade periods could be of thermomagnetic origin, while mid-term volcanomagnetic variations associated with the cycles of Merapi activity could be of piezomagnetic origin. Short-term variations of a few weeks duration, less than 1.5 nT, are well correlated with the 1995–1996 seismic activity.