Geology, tectonics and post-2001 eruptive activity interpreted from high-spatial resolution satellite imagery : the case study of Merapi and Seremu volcanoes, Indonesia

Remote sensing has long been recognized as a tool for analysis at active and hazardous volcanoes because it can augment our understanding of the processes that underlie volcanic activity so as enable us to apply this understanding to volcanic risk reduction. This thesis presents a volcanological study using High-Spatial Resolution optical images (IKONOS, Pleiades, GeoEye, Quickbird and SPOT5 satellites), radar data (ALOS-PALSAR sensor) and thermal (ASTER satellite and MODIS hot spot) images. In association with DEMs and low-altitude aerial photographs, remote sensing techniques have been applied for tracing the evolution of activity at Semeru and Merapi, two of the most active and densely populated volcanoes in Java, Indonesia. This remotely sensing-based study has unraveled structures, geological features and erupted deposits of both volcanoes and has improved the existing hazard assessment after their most recent eruptions. The thesis also presents the first advance towards deciphering possible interactions between regional tectonic earthquakes and renewed stages of eruptive activity of Merapi and Semeru volcanoes based on the analysis of volcanic hotspots detected by the MODVOLC technique.The geological map of Semeru is updated, including additional data derived from the interpretation of the most recent satellite images, aerial photographs, DEM analysis and fieldwork. The post-2001 eruptive activity at Semeru, including the large PDC-forming eruption in 2002-2003 and uncommon lava flow eruptions in 2010-2014 are investigated. The fact that Semeru has produced several lava flows from the central summit vent between 2010 and 2014 may herald a profound change in eruption style for the first time since at least 1967. At the time of writing, a dome-fed coulee in the Jonggring-Seloko crater continues to grow and lava flows are extending to distances of >2 km down Semeru's SE-scar; their fronts may collapse and produce large-volume pyroclastic density currents (PDCs), perhaps exceeding the average (1967-2007) volume range of 3 to 6.5 million m3. Future dome-collapse PDCs may travel farther down the main SE scar and can spill over its lowermost rims towards the southwest and eastward radiating drainage network. The 26 October-23 November 2010 eruption was the Merapi’s largest event since 1872 (it attained VEI=4). The interpretation of HSR images shows that due to the explosive eruptions, the summit area lost about 10 x 106m3 and the SSE-trending Gendol Breach enlarged to reach 1.3 x 0.3 x 0.2 km in size. The new, enlarged and deep summit crater including the 2010 lava dome is extremely unstable having been weakened by the post-2010 explosive events. This instability is a result of the steep Gendol Breach below the mouth of the crater and the steep and unstable crater walls. The 2010 Merapi pyroclastic and lahar deposits have been identified by applying several classification methods to HSR optical images and dual-polarization synthetic aperture radar (SAR) data. The results show the ability of remotely sensed data to capture the extent and impacts of pristine deposits shortly after emplacement and before any reworking, and highlight the purpose of using high-spatial resolution imagery and SAR data on persistently active volcanoes where access for field survey is often impossible. The 2010 tephra and PDC deposits covered ca. 26 km2 in two catchments of Gendol and Opak Rivers on Merapi’s south flank, i.e. 60-75% of the total PDC deposit area and a total bulk volume of 45 x 106m3. The tephra-fall deposit covered an area of ca. 1300 km2 with a volume range of 18-21 x 106m3. Volumes of these deposits were estimated using the areas determined from remote sensing data and deposit thickness measured in the field. (...)