GPR Facies Analysis of Block-and-ash Flows at Merapi Volcano, Central Java, Indonesia

Merapi, a 2911-m-high basaltic andesite volcanic complex in Central Java is one of the most frequently erupting volcanoes in Indonesia and best known for its nearly persistent volcanic activity characterized by the extrusion of viscous lava domes and collapse of these domes to produce block-and-ash flows (BAFs). During the most recent eruptive episode in 2006, BAFs affected the densely populated areas on the volcano's southern flank and were the first major flows in this area for over a century. More importantly, the flows were not confined to the existing river valleys but spilled over the valley sides to create overbank flows that resulted in fatalities in the village of Kaliadem about 5 km away from Merapi. In August 2008, a four-week field programme of ground penetrating radar (GPR) data collection was carried out on the 2006 deposits aiming to improve traditional models of BAF transport and deposition on the basis of observational and interpretational evidence, which are often complicated by a combination of poor exposure, rapid lateral facies variations and unknown palaeo-topography. The GPR survey focused on the large-scale (deposit) and small-scale (intra-deposit) structures of (a) the valley-filling BAF deposits in the proximal, medial and distal regions of the Gendol river valley and (b) the overbank deposits in the Kaliadem area. The GPR sections of the valley-filling deposits are characterized by a high density of diffraction hyperbolae from large, decimetre- to metre-sized blocks within the deposits, either randomly distributed or concentrated in particular horizons within the deposits, which allow larger scale variations in grain size and deposit thickness as well as flow unit boundaries to be traced over areas where exposure is limited or absent. Similar patterns are observed in the GPR sections of the proximal, wedge-shaped overbank deposits, though the deposits lack the abundance of large blocks typical of the valley-filling deposits. Distinct, coherent and traceable reflections are evident in the GPR sections of the more distal overbank deposits that relate to defined, sharp changes in matrix grain size, distinct blocky horizons and internal architectures that can be related to observable units in exposed deposit sections. These variations, indicative of subtle, yet important, changes in the nature and dynamics of the overbank flows across interfluves, occur over relatively short flow distances (100 metres or more) that are clearly mapable in the GPR sections at a metre scale. Given the unpredictable behaviour and potential for loss of life of these overbank flows, not only at this volcano but also at similar locations around the world, it is important that these particular deposits are studied in detail. This study shows that GPR is a useful, non-invasive tool for developing new, improved interpretations on the transport and emplacement dynamics associated with BAF deposits.