ABOVE: The New Guinea Trench marks the boundary between two plates. The Pacific Plate slides obliquely under the Australian Plate at a rate of 11 cm per year. The grinding together of the two plates gives rise to shallow earthquakes along the trench axis and as far south as the Bewani and Torricelli Ranges. A magnitude 7.9 earthquake on the inner wall of the trench in September 1996 caused a tsunami that took 107 lives on Biak Island.
BELOW: The cross-section across the New Guinea Trench is based on a seismic reflection profile. It shows the relatively steep slope of the inner wall of the trench, offshore fron Sissano, and a number of faults. Probably it was movement on one of these faults that caused the 17 July earth-quake and tsunami. The earthquake may have caused sediments to slump down the slope. The added effect of the sudden displacement of the ocean water by the sediments could explain the unusual power of the tsunami.