Temporal gravity at Merapi during the 1993–1995 crisis: an insight into the dynamical behaviour of volcanoes

Gravity signals were recorded during volcanic activity of Merapi volcano between 1993 and 1995. First we repeated measurements of gravity (with Scintrex CG3-M) and of elevation (with GPS receivers) on a network. Within this period, we observed little deformation (less than 5 cm), but significant gravity changes (up to almost +400 μGal and −270 μGal). The growth of the dome explains most of the gravity signal. However, a residual gravity change is interpreted as an increase of 4×108 kg of the volcano mass, under the northwestern part of the summit. We consider and discuss several models, including non-volcanic effects (e.g. water table level change), and magmatic processes, such as the intrusion of magma, or the effect of crystallisation of implaced magma. Secondly, we installed a gravity meter in Babadan observatory, located at 4 km from the summit, which has been recording continuously since 1993. The analysis of these records leads to a precise Earth tide model for the Merapi area with an accuracy of 1.3 μGal for M2. We consider both the continuous monitoring of the mass movement within the volcano and the response of the volcano to the tidal potential. A correlation exists between residual drift and seismic and volcanic activity. For example, a decrease of the residual drift corresponds to intensive seismic activity (LF event) and the occurrence of nuées ardentes, whereas no remarkable activity is associated with increases of the residual drift. The admittance variations, a combination of the meter sensitivity (which is tilt-dependent) and the mechanical response of the ground to tidal forces, are also correlated to volcanic events. We propose that during 1993–1995, oscillations of the internal pressure (a few MPa) due to crystallisation and degassing magma lead to direct summit activity.