Mechanical discontinuities monitoring at Merapi volcano using kinematic GPS

Merapi volcano (Java, Indonesia) is in almost continuous activity with growth of an andesitic lava dome. This dome frequently collapses to form potentially deadly glowing avalanches, explosions and nuées ardentes. To monitor the evolution of surface displacements and to model the associated magmatic sources, we established a Global Positioning System (GPS) network in 1993 and have measured it each year using the static GPS method. However, the limited number of benchmarks and the geometry of the network did not allow us to precisely locate major mechanical discontinuities within the edifice. Precisely locating these discontinuities is of central importance because they delimit areas of potential instability and provide means to evaluate potential volumes of falling material.The kinematic GPS method offers a way to partially solve the problem of temporal and spatial sampling of the displacement field, but its accuracy is usually insufficient to monitor small displacements. We propose here a strategy of field measurements and adjustments which combines kinematic positioning (1 min) and rapid static baselines (15 min) to get a 1.5-cm error (95% confidence). At Merapi summit, we have installed about 50 benchmarks covering the area around the main crater. Field measurements of this new network with our method take a few hours and the data processing has been automated.We present the results of 8 surveys from 1999 to 2002, a period that includes a dome collapse in January 2001. Our results show large horizontal displacements towards the northwest, starting in July 2000 and reaching about 50 cm in amplitude in November 2000 that we interpret as precursors to the dome collapse. We also locate two presently active discontinuities at the summit of the volcano. This approach can be implemented easily on other active volcanoes.