Home / News and Events / Media Releases / Physical evidence of tsunami

Physical evidence of the 17th July tsunami - 07/10/1998

A tsunami with wave heights locally exceeding 15 m was generated by a Ms 7.1 earthquake on 17th July 1998, near Sissano Lagoon, on the northwestern coast of Papua New Guinea.

The true death toll from this tsunami will never be known, but it is thought to exceed the official count of 2189 (based on a 1990 census) by more than a factor of two. Most of the deaths were in the villages of Warapu and Arop, situated on the narrow sand spit fronting the Sissano Lagoon, where only 800 survived from an estimated population of 4000-5000 people.

No structures were left standing in this area which was hit by a fast moving wall of sand-laden water up to 15 m high, within about two minutes of the earthquake. The height of the tsunami is anomalous for the earthquake magnitude, but the region of extreme damage (total destruction of all buildings) along the coastline is only some 19 km, as opposed to an affected coastline of some 40 km.GPS locations, morphology, evidence of wave height, and the depth of deposition and scour were measured along four transects across the Warapu spit. Sediment samples were taken along each transect and additional longshore sites were also linked into the GPS grid. A 1.5 m core was taken at the landward end (on the lagoon edge) of transect 2. Initial results indicate a maximum wave height of 17.5 masl (Transect 3), with deposition exceeding 1.0 m on the seaward beach ridge of Transect 2. Scour in excess of 2 m was recorded in the vicinity of the graveyard.

Evidence for wave height included broken branches (unreliable), debris snagged in trees (reliable), stripped bark and tree damage (reliable), and material pushed again tree trucks (moderately reliable). A preliminary analysis of core data indicates at least two erosional contacts, one overlain by a fining-upwards sequence of sand.

The unusually high tsunami waves appear to have been focussed onto the Sissano Lagoon area by coseismic coastal subsidence (identified in the field). Preliminary modelling using the USGS Earthquake Database record (2.93oS, 141.80oE Ms 7.1, Epicentre depth: 6.0 km, 5o Dip), indicates that a 2 m horizontal displacement, approx. 40 cm subsidence (landward) and 50 cm uplift (seaward) generates the wave type characterised by the physical evidence recorded.

Photographs show some important locations in the area.

Warapu side of the spit – Sissano Lagoon. Debris stranded by the waning flow of the backwash.

Warapu side of the spit – Sissano Lagoon. Debris stranded by the waning flow of the backwash.

The classic photo of the bucket in the tree – Arop village side. However, it should be noted that the tree is now submerged and dying. This is indicative of subsidence at the site.

The classic photo of the bucket in the tree – Arop village side. However, it should be noted that the tree is now submerged and dying. This is indicative of subsidence at the site.

Arop village site – no buildings visible, signs of scour and subsidence on landward side of lagoon (to the right).

Arop village site – no buildings visible, signs of scour and subsidence on landward side of lagoon (to the right).

Bent bicycle amongst the wreckage – Warapu vilage site.

Bent bicycle amongst the wreckage – Warapu vilage site.