Where it is and why it is separate
The North Bismarck Plate covers about 388,000 km² of the northern Bismarck Sea, north of New Guinea and New Britain. Only about 2.3% of Papua New Guinea sits on it, mainly small islands in the Admiralty group around Manus.
Geologists separate it because a sharp band of earthquakes, the Bismarck Sea Seismic Lineation, cuts straight across the Bismarck Sea. North of that line the seafloor moves almost with the Pacific Plate; south of it, the South Bismarck Plate moves very differently. Because the North Bismarck Plate moves so slowly relative to the Pacific, many simplified maps do not show it at all.
What happens at its edges
The southern boundary with the South Bismarck Plate is the longest, at 1,217 km. It is mostly a transform fault, with short spreading and collision segments, and relative motion averages about 13 cm/yr, very fast for a transform boundary.
To the north and east, the boundary with the Pacific Plate is slow convergence of about 0.9 cm/yr. To the northwest, it meets the Caroline Plate along the Manus Trench at about 1.3 cm/yr. In the middle of the Bismarck Sea, the tiny Manus Plate sits between the two Bismarck plates, opening the Manus Basin.
At its southeastern corner, two short subduction boundaries carry the Solomon Sea Plate and the Woodlark Plate beneath it, at about 8.2 and 7.8 cm/yr. This corner, where the New Britain Trench bends toward Bougainville, is where almost all of the plate’s great earthquakes happen.
Earthquakes and volcanoes
The dataset lists 79 earthquakes of magnitude 6.5 or larger since 1900, three of them magnitude 8 or more. The 1919 Papua New Guinea earthquake (M8.15) is the largest. In July 1971, the New Britain earthquake (M8.0) and the Bougainville earthquake (M8.1) struck 12 days apart. More recently, an M7.9 quake in December 2016, about 140 km east of Kokopo, struck 95 km deep. Several of these quakes broke inside the sinking Solomon Sea slab rather than on the shallow plate boundary.
Volcanoes are scarce. The dataset lists two, including the St. Andrew Strait volcanic complex near Manus Island, which last erupted in 1957.