The Sissano, Papua New Guinea tsunami of July 1998 - offshore evidence on the source mechanism

The source of the local tsunami of 17th July 1998 that struck the north sho re of Papua New Guinea remains controversial, and has been postulated as du e either to seabed dislocation (fault) or sediment slump. Alternative sourc e mechanisms of the tsunami were addressed by offshore investigation using multibeam bathymetry, sub-bottom profiling, sediment sampling and observati on from the JAMSTEC Dolphin 3 K Remotely Operated Vehicle and Shinkai 2000 Manned Submersible. The area offshore of Sissano is a complex active conver gent margin with subduction taking place along the New Guinea Trench. Domin ant transpressional convergence results in diachronous collision of the hig hstanding North Bismarck Sea Plate in a westerly direction. The result is a morphological variation along the Inner Trench Slope, with the boundary be tween eastern and western segments located offshore Sissano in an area of o n- and offshore subsidence. This subsidence, together with nearshore bathym etric focusing, is considered to increase the tsunamigenic potential of the Sissano area. The offshore data allow discrimination between tsunami gener ating mechanisms with the most probable source mechanism of the local tsuna mi as a sediment slump located offshore of Sissano Lagoon. The approximatel y 5-10 km(3) slump is located in an arcuate, amphitheatre-shaped structure in cohesive sediments that failed through rotational faulting. In the area of the amphitheatre there is evidence of recent seabed movement in the form of fissures, brecciated angular sediment blocks, vertical slopes, talus de posits and active fluid expulsion that maintains a chemosynthetic vent faun a. Dating of the slump event may be approximated by the age of the chemosyn thetic faunas as well as by a seismic signal from the failing sediment mass . Faults in the area offshore Sissano are mainly dip-slip to the north with recent movement only along planes of limited lateral extent. A possible th rust fault is of limited extent and with minimal (cm) reverse movement. Fur ther numerical modelling of the tsunami also supports the slump as source o ver fault displacements. (C) 2001 Elsevier Science B.V. All rights reserved .