Geohazards in Indonesia: Earth science for disaster risk reduction – introduction

Indonesia encompasses one of the most active tectonic regions on Earth. Its territory includes over 18 000 km of major tectonic plate boundary, more than twice that of Japan or Papua New Guinea (Fig. 1). This includes: the Great Sumatran Fault, at 1900 km length about 50 per cent longer than the San Andreas and North Anatolian faults; a 6000 km length of convergent plate margin, stretching from off Sumatra to the Banda Sea that has experienced the world’s second largest megathrust earthquake (Stein & Okal 2005) and its largest intraslab earthquake (Osada & Abe 1981); and one of the highest-curvature subduction zones in the world, with the Banda Arc curving through 1808 in 2200 km of its length. Indonesia is home to at least 127 active volcanoes, more than any other country in the Asia-Pacific. These eruptions include three of the largest and deadliest in human history: 1257 Rinjani (VEI (Volcanic Explosivity Index) 7, fatalities unknown; Lavigne et al. 2013), 1815 Tambora (VEI 7, 71 000 fatalities; Oppenheimer 2003), 1883 Krakatau (VEI 6, 36 000 fatalities; Self & Rampino 1981). Other remarkable features of Indonesia’s tectonic environment are the divergent double subduction of the Molucca Sea Plate resulting in arc–arc collision (Widiwijayanti et al. 2003); the subduction of continental lithosphere in the Banda Sea arc–continent collision (Fichtner et al. 2010); one of the world’s youngest orogenies along the Timor–Tanimbar section of the Banda Arc (Kaneko et al. 2007; Keep & Haig 2010); and areas of pronounced mud volcano activity (Milkov 2000). Indonesia’s pronounced tectonic activity poses a grave threat to its large and often vulnerable population. By population Indonesia is the fourth largest country in the world, with a population expected to grow from 238.5 to 305.7 million between 2010 and 2035 (Indonesian Bureau of Statistics 2014). Almost 50 per cent of Indonesians live on the island of Java, the world’s most densely populated island. Java accounted for over 60 per cent of Indonesia’s urban land use in 2010 and is expected to accommodate 80 per cent of Indonensia’s urban expansion between 2000 and 2030 (Muis et al. 2015). As noted for buildings in Banda Aceh following the 2004 Great Sumatra Earthquake (Saatcioglu et al. 2006), urban residential construction in Indonesia often comprises low-rise, non-engineered structures that are unable to withstand the loads that might be placed on them by a major event such as an earthquake, tsunami or volcanic eruption. From the combination of increased population, greater urbanization and poor construction standards, we can expect that Indonesia’s vulnerability to natural hazards is high, much higher than it has ever been in the past. Because geological hazards in Indonesia comprise a potent threat to a large and vulnerable population, it is important that decisions made by disaster managers be informed by the best available Earth science. Considerable progress has been made in recent years in providing the Earth science that can support disaster risk reduction, and it is the purpose of this Geological Society of London Special Publication to document some of this progress.