Monitoring the UPS and Downs of Sumatra and Java with D-Insar Time-Series

We performed, for the first time, a global D-InSAR survey of the Indonesian islands of Sumatra and Java to define locations where deformation is occurring. The goals of this study are 1) to create an inventory of actively deforming volcanic centers and 2) monitor all types of ground motion. This work provides ground deformation data for previously unmonitored areas and can assist the Indonesian authorities to improve hazards assessment. The D-InSAR survey covers an area of about 500 000 km2 and 3000 km long on the islands of Sumatra, Java and Bali. We used ALOS data from 45 tracks and more than 1500 granules obtained from the Alaska Satellite Facility (ASF) through the US Government Research Consortium (USGRC). We completed more than 1000 interferograms spanning a period from the end of 2006 to the beginning of 2009. L-band SAR images enable deformation mapping at global scales even in highly vegetated areas where C-band signal experiences loss of coherence. To identify locations where ground deformations are occurring, we used multiple SAR acquisitions of the same area and performed time series analysis using the Small BAseline Subset (SBAS) method. Interferograms with a maximum spatial baseline of 3000 m were phase-unwrapped and subsequently inverted for the phase with respect to the first acquisition. Temporal coherence of each pixel is computed on the set of interferograms in order to select only pixels with high temporal coherence. The compiled InSAR velocity map reveals the background level of activity of the 84 volcanic centers constituting the Sumatra, Java and Bali volcanic arcs. We identified possible uplift at 6 volcanic centers: Agung (Bali), Lamongan (Java), Lawu (Java), Slamet (Java), Kerinci (Sumatra) and Sinabung (Sumatra). Moreover, we identified subsidence in 5 major cities and 1 coastal area. Subsidence rates range from 6 cm/yr in Medan, the largest city of Sumatra, to more than 15 cm/yr in Jakarta. These major subsidence areas are probably due to ground water extraction needed to support the increasing population and industrial activities.