Toward a revised hazard assessment at Merapi volcano, Central Java

Of 1.1 million people living on the flanks of the active Merapi volcano, 44 0,000 are at relatively high risk in areas prone to pyroclastic flows, surg es, and lahars. For the last two centuries, the activity of Merapi has alte rnated regularly between long periods of viscous lava dome extrusion, and b l ief explosive episodes at 8-15 year intervals, which generated dome-colla pse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26-54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 repo rted eruptions since the mid-1500s killed about 7000 people. The current ha zard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, term ed 'forbidden zone', 'first danger zone' and 'second danger zone', based on successively declining hazards. Revision of the hazard map is desirable, b ecause it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be: swept by lahars, and excludes: some a reas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorpo rate social, technical, and economic factors of vulnerability. Eruptive hazard assessment at Merapi is based on reconstructed eruptive his tory, on eruptive behavior and scenarios, and on existing models and prelim inary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruption s, helps to define the extent and recurrence frequency of the most hazardou s phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows t raveled as far as 9-15 km from the sourer, pyroclastic surges swept the fla nks as far as 9-20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyaharta 25 km to the south, and subsequent lahars spi lled down the radial valleys as far as 30 hm to the west and south. At leas t one large edifice collapse has occurred in the past 7000 years (Newhall c t al., 2000; Camus et al., 3000). Secondly, four eruption scenarios are por trayed as hazardous zones on two maps and derived from the past eruptive be havior of Merapi and from the most affected areas in the past. Thirdly, sim ple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclas tic and lahar flowage on the Banks and in radial valley channels, and to ou tline areas likely to be devastated. Three major threats are identified: (1) a collapse of the summit dome in th e short-to mid-term, that can release large-volume pyroclastic flows and hi gh-energy surges towards the south-southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flan ks of Merapi at least once every century; (3) a potential collapse of the s ummit area, involving the fumarolic field of Gendol and part of the souther n Rank, which can contribute to moderate-scale debris avalanches and debris flows. (C) 2000 Elsevier Science B.V. All rights reserved.