Monitoring ground deformation at the decade volcano Gunung Merapi,Indonesia

Pressure changes within a volcano lead to deformations of his edifice. To discriminate local and regional ground movements, four deformation stations with clusters of continuous borehole tiltmeters have been installed at the hillsides of Gunung Merapi. In addition, recording local environmental parameters allows the recognition of interfering local tilt signals. First analyses of the data records show: The significant tidal variations of the tilt signal validate the successful coupling between the instruments and their surroundings. Transient tilt signals correlated to rainfalls are probably caused by poroelastic deformation of the hillsides. Four volcanic events occurred during the observation period; all of them were accompanied, if not triggered, by heavy rainfalls. Tilt anomalies, considerably larger than the rain induced tilts, were recorded around the event of October 31, 1996, coincident with an extraordinary high seismic activity. If this activity reflects internal fracture processes prior to the eruption, the accompanying deformations might partially explain the observed tilt signal.