Field survey on the 1998 tsunami in the northwestern area of Papua New Guinea

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The first investigation on the tsunami caused by the earthquake offshore northwestern coast of Papua New Guinea (PNG) was carried out by the International Tsunami Survey Team (ITST) during the period of 31 July to 7 August 1998. On the evening of Friday 17 July a massive tsunami swept across the sand bar that forms the outer margin of Sissano Lagoon, west Sepik, PNG. Initial media reported that the tsunami struck west of the town of Aitape in the west Sepik province, hitting at least four villages. And the was 7 - 10 m and that up to 3,000 persons were killed or missing. This seemed to be an unusually damaging tsunami given the size of the earthquake (M=7). Members of the ITST decided that a field survey was necessary as soon as possible to try and determine the true value of the maximum run-up and to accurately map the run-up distribution along the coast. The survey to the damaged area confirmed the 7 - 10 m wave heights reported and found the place where the waves were larger up to 15 m and extreme overland flow velocities of 10 to 15 m/s. The severe damage and extreme wave heights were confined to a relatively short (30km) stretch of coast between Aitape and Sissano Villages and specially large along the sand bar the outer margin of Sissano Lagoon. These suggest that tsunami energy is concentrated in this narrow region. Although a tsunami is to be amplified in V-shaped bay, peninsula, island and ridge, the coastal line in Sissano is straight and the contour lines of water depth are almost parallel. Extremely large tsunami than estimated by an earthquake (M=7) might be caused by not a local bottom topography but a tsunami generation. Eyewitness of a loud bang, hot water, bubbles and foul smell suggest sea landslide or a gas explosion offshore triggered by the earthquake as a secondary mechanism of tsunami generation. Further field investigation and research on it are required to solve a riddle of the tsunami generation.