Assessing the Pyroclastic Density Current Hazards at Merapi: From Field Data to Numerical Simulations and Hazard Maps

Merapi is considered as the most active and the most frequently eruptive volcano in Indonesia and is responsible for more than 5200 casualties since the eighteenth century, mainly caused by pyroclastic density currents (PDCs). Although eruptions are predominated by gravitational dome-collapse events and small explosions of VEI < 3, some PDCs associated with larger explosive events repeatedly reached distances up to 15-20 km from the summit during the last 200 years. PDC hazard mapping in Indonesia is traditionally based on the maximum extent of PDCs as derived from previous eruptions. The potential hazard of long-runout, widespread high-energy PDCs able to spread across densely populated interfluve (non-valley) and distal regions is now considered the most challenging component of the PDC hazard mapping at Merapi. PDC hazard modelling approaches have developed and diversified in several ways and can be subdivided into either deterministic (i.e. scenario-based) or probabilistic types. The depth-averaged modelling approach seems to be suitable for the simulation of flows and surges generated by dome collapses, the most common type of PDCs at Merapi. However, regardless of the modelling approach chosen, simulation results should always be interpreted carefully and, if integrated into a hazard plan, done with expert advice. In doing this, the use of well-constrained geological data, validation metrics and statistical approaches such as those described here can provide valuable insight and assist in the PDC hazard analysis process. Outcomes of such modelling efforts could provide the basis for establishing an interpretation framework and defining the 'best practices' to conduct rigorous PDC hazard assessments. Such guidelines could be widely distributed to correctly inform and advise geological surveys worldwide about the breadth and depth of understanding of methodologies and procedures currently available for undertaking robust PDC hazard assessments at other active volcanoes like Merapi.