Rabaul Caldera, Papua New Guinea: Volcanic hazards, surveillance, and eruption contingency planning

Abstract

Planning for volcanic contingencies at the Rabaul caldera complex is a difficult task, requiring urgent attention in view of the results of volcano surveillance and historical eruption frequency. Historical eruption periodicities have ranged from about 2 to 59 years; the most recent eruptions were in 1937–1943 and caused more than 500 fatalities in May 1937. Surveillance since 1971 has revealed uplift of part of the caldera floor and increasing seismicity in a ring structure believed to define the caldera bounding fault or faults. These phenomena are interpreted as the prelude to a possible eruption. Potential eruption sites are all close to Rabaul town and outlying residential areas, and include the volcanoes of Sulphur Creek (last active about 1850), Vulcan and Tavurvur (1937–1943), and Rabalanakaia, but new sites should also be considered in relation to the results of volcano surveillance. A wide range of volcanic hazards accompanying different scales of eruption must be considered for the Rabaul area. The most likely types of eruption may be similar to the relatively small events that have been witnessed over the last century whose hazards included air-fall tephra, pumice rafts, pyroclastic flows and surges, volcanic earthquakes, volcanic-gas discharges, tsunamis, lightning strikes, and the fall of mud-rain, torrential run-off, and mudflows as a result of accompanying rain storms. Other potential eruption types of much larger scale could involve cauldron subsidence which could cause destructive tsunamis and could be accompanied by explosive activity of major proportions. Air-fall tephra is considered to be one of the main hazards from a future eruption, and its effects in and around Rabaul are primary influences in the choice of evacuation routes and points of refuge. Hence, theoretical tephra dispersal patterns have been considered in some detail. Air-fall tephra deposition from Rabaul eruption clouds at heights of less than about 4 km is determined by a biannual change in wind direction from southeast in May to October/November (trade winds) to northwest during the monsoon from December to March/April. In contrast, clouds that rise higher than about 4 km are blown by east winds all year round. Another principal hazard is pyroclastic flows and surges, which took place during the 1937 Vulcan eruption, but have until recently been overlooked, although flows or surges (or both) are clearly distinguishable on eruption photographs. The lack of suitable evacuation routes and problems in choosing safe and reasonably accessible refuge points compound the difficulties in preparing a practicable contingency plan. However, in small—moderate eruptions the safest areas around Rabaul are considered to be at the Duke York Islands, about 30 km to the east, and at Cape Gazelle, about 25 km to the southeast. Seaborne evacuation to main ports in central New Britain or on the neighbouring islands of New Ireland and Bougainville, or overland evacuation using a southwards-expanded road system, would be necessary in the event of forewarning of a major eruption.