Where it is and what it carries
The South Bismarck Plate covers about 309,000 km² in northern Papua New Guinea. It holds about 16% of the country’s land: the island of New Britain, the Huon Peninsula and the Finisterre Range on the mainland, and a chain of volcanic islands along the north coast. The rest is the floor of the southern Bismarck Sea.
The plate’s western end is caught in a collision between the Finisterre volcanic arc and mainland New Guinea that is still under way. Coral reef terraces on the Huon Peninsula have been lifted far above sea level as the arc pushes into the mainland.
What happens at its edges
The longest boundary, 1,217 km, separates it from the North Bismarck Plate along the Bismarck Sea Seismic Lineation. This is mostly a transform boundary, and the plates slide past each other fast, at about 13 cm/yr on average.
To the south, the Solomon Sea Plate dives beneath New Britain at the New Britain Trench. Relative speeds average about 11.3 cm/yr and reach about 15.8 cm/yr, among the highest in the PB2002 model. Subduction here feeds the volcanoes along New Britain.
In the northeast, the plate spreads away from the tiny Manus Plate along the Manus Basin at about 8.5 cm/yr on average. In the west it meets the Woodlark Plate along the Ramu and Markham valleys, where collision, rifting and strike-slip faulting combine at about 3.5 cm/yr.
Earthquakes and volcanoes
The dataset lists 141 earthquakes of magnitude 6.5 or larger since 1900, two of them magnitude 8. The 1906 Papua New Guinea earthquake (M8.0) struck near the Huon Gulf. In November 2000 the New Ireland earthquake (M8.0) was followed within a day by two M7.8 quakes southeast and south-southwest of Kokopo. An M8.0 quake also struck southeast of Kokopo on 1 January 1916. In 2022 an M7.6 quake hit 116 km beneath the mainland near Kainantu.
With 24 volcanoes in the dataset, this small plate is one of the most volcanic places in the southwest Pacific. Manam, Langila and Ulawun have erupted recently. Rabaul, a caldera at the northeast tip of New Britain, last erupted in 2014. In 1888 part of Ritter Island collapsed into the sea and set off a destructive tsunami. Long Island, Karkar and Kadovar are also active.