Using topographic signatures to classify internally and externally driven tilt anomalies at Merapi Volcano, Java']Java, Indonesia

A 3-dimensional Finite-Element-Model of Merapi Volcano was developed to calculate the surface deformation field in response to internal pressure variations. The model geometry is based on a new Digital Elevation Model with a grid size of 15 m and an accuracy of 7 m. This allows an investigation of local effects due to radially striking features like deeply eroded canyons, separated by narrow ridges. Close to the ridge margins, topographic disturbances are invoked with amplitudes comparable to the expected signal on a smooth surface. Comparison of the model results with observed tilt records at four locations reveals that topographically induced tilts exhibit a certain pattern that differs systematically from the spatial characteristics of tilt disturbances induced by meteorology. We conclude that a monitoring of the difference of two instruments located at opposite ridge margins provides a simple test to discriminate externally from internally driven tilt anomalies.