The 17 July 1998 tsunami, Papua New Guinea: evidence and initial interpretation

A M-w 7.1 earthquake that occurred at 08:49 hours, 17 July, 1998 UTC (6:49 PM local time) off the north coast of Papua New Guinea generated a locally destructive tsunami, More than 2189 people died, and virtually no structures were left standing along 19 km of coast. A fast moving wall of sand-laden water left fishing nets and other detritus in trees up to 17.5 m above sea level. Concrete was stripped to the reinforcing metal, and some trees were ripped out and carried more than a kilometre back into Sissano Lagoon. The site is in an active sedimentary basin, the Aitape Trough with 4500 m thickness of Neogene sediments, between the Bewani fault zone and the Wewak Trench. There is some debate about the form of tsunami propagation for this event. However, we present evidence for subsidence of similar to 30-40 cm on the landward side of the spit fronting Sissano Lagoon. Furthermore, the area may have subsided up to three times this century. In situ stumps of drowned trees in the lagoon record at least one earlier event. The low-angle Harvard University CMT solution (M-w 7.1,depth similar to6.0 km, similar to 10 degrees landward dip on fault with northward displacement) is consistent with the tectonic setting and pattern of aftershocks. Elastic modelling of the energy release suggests similar to2 m horizontal displacement over 600 km(2), similar to 40 cm subsidence (landward) and similar to 60 cm uplift (seaward). Convergent flow of the displaced water into the area of subsidence focused wave energy on the coast and generated the locally high wave. The record of tsunami at Sissano Lagoon indicates that the spit fronting the lagoon is unsafe for habitation. There is potential for similar coseismic coastal subsidence to focus tsunami in other areas with similar tectonic settings. (C) 2000 Elsevier Science B.V. All rights reserved.