Interpretation of ground surface changes prior to the 2010 large eruption of Merapi volcano using ALOS/PALSAR, ASTER TIR and gas emission data

• A combined analysis of D-InSAR displacement, SAR backscattering intensity, and ground surface temperature
• Inter-eruptive magmatic period at Mt. Merapi
• Cyclic episode of Mt. Merapi deformation
• Successive movement of individual magma batches

Understanding precursory signals is essential to forecast eruptions hazards and mitigate risks. Satellite observations have been shown to increasingly contribute to this goal. We use Differential-Inteferometric Synthetic Aperture Radar (D-InSAR) and SAR backscattering intensity data from ALOS/PALSAR to define a time-series of ground surface displacements for the 2006–2010 inter-eruptive period at Merapi volcano. We correlate trends in the displacement data to trends for summit temperature (determined using data from the ASTER sensor) and to gas emission data. We show that processing of the satellite data must be performed carefully before meaningful interpretations can be drawn. For example, after careful removal of the topographic effect on phase delays, we detect only subtle inter-eruptive episodes of cyclic deformation in the D-InSAR and SAR backscattering data. These small (mostly sub-centimeter) displacements contrast with meter-scale pre-eruptive displacements along an electronic distance measurement (EDM) line on the south flank of the volcano. We suggest that this difference is a consequence of localized movement within the summit area — a result that has important implications for understanding Merapi's structure, for monitoring network design and for eruption forecasting. Although small, Merapi's inter-eruptive deformation cycles are also seen in thermal data from ASTER and gas emissions. Accordingly, we interpret these cycles as the result of successive movement of individual magma batches migrating upward from deeper to shallower storage systems located along a NE dipping conduit. In view of magma migration prior to eruptions, these cycles may also serve as a precursory signal for large eruptions at Merapi, such as the one that took place in early November 2010.