Electromagnetic evidence for an ancient avalanche caldera rim on the south flank of Mount Merapi, Indonesia

Long-Offset Transient Electromagnetic (LOTEM) data and VIBROTEM data from the south flank of Mount Merapi on Java island, Indonesia, are interpreted with one-dimensional (1D) inversions as well as two-dimensional (2D) forward modelling. One-dimensional joint inversions of several components of the electromagnetic field with Occam's method reduce the number of equivalent models, which were derived from inversions of single components and fit the data to a similar misfit. The 1D results, together with results from other geophysical measurements, serve as the basic model for further 2D forward modelling. The final model depicts a layering that follows the topography of the strato-volcano. In the depth range of 500 m to 1000 m, the resistivity of the layers decreases rapidly downwards into a good conductor with resistivities below 10 Ωm. The deepest layer has a resistivity of 0.4 Ωm which is quantitatively explained with a combination of saline fluids and hydrothermally altered minerals. Furthermore, the final model supports a hypothesis from the interpretation of central-loop TEM (Transient Electromagnetic) data that there is a fault structure below the southern flank, approximately 7.3 km south of the summit. To the north of the fault, the top of the good conductor is lowered from a depth of 500 m to 1000 m. We propose that the fault structure coincides with an ancient avalanche caldera rim.