Introduction
The South Bismarck Plate is a small lithospheric block beneath the southern Bismarck Sea in the southwestern Pacific. It incorporates fragments of continental and island crust, most notably the eastern sector of New Guinea and the island of New Britain, which together constitute the plate’s principal surface expression. Tectonically active, the plate experiences frequent earthquakes and volcanism indicative of intense crustal deformation and magmatic activity concentrated along its boundaries. Much of this behavior is driven by the New Britain subduction system, where convergent interactions generate subduction-related seismicity and volcanic arcs through arc–continent and plate–plate convergence. Situated within the Pacific Ring of Fire, the South Bismarck Plate’s high levels of seismic and volcanic activity make it a significant component of regional geohazard assessments.
Tectonics
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The South Bismarck microplate occupies a central position north of the Australian plate boundary within the tectonically intricate New Guinea–Melanesia region, abutting the Australian plate to the south, the Pacific plate to the east and northeast, and a number of smaller plates and fragments scattered across the Bismarck Sea and adjacent basins. Earlier regional reconstructions (notably the 2003 model) contain mislabeling and inaccurate plate extents—especially for the Woodlark plate—because they predate later observations; accordingly some inferred boundaries and sizes in that model should be regarded as provisional.
Convergence dominates the microplate’s southern margin. Southward subduction along the New Britain Trench has driven arc construction that produced New Britain and parts of the Solomon Islands and concentrates seismicity around New Britain. Immediately to the south the Solomon Sea plate subducts beneath New Britain, and a probable active Trobriand plate with inferred fault-zone relations further complicates the local subduction–collision regime.
Contemporary GPS kinematics show the South Bismarck block is being driven northward by the northward-moving Australian plate, while the area historically identified as the North Bismarck plate translates westward under the influence of the Pacific plate. The so-called North Bismarck shows no detectable independent motion relative to the Pacific and is therefore treated as a relic; because its motion closely matches that of the Pacific plate, much of the Melanesian arc east of New Ireland can be regarded as effectively attached to the Pacific plate.
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The boundary between the North and South Bismarck blocks—the Bismarck Sea Seismic Lineation (BSSL)—is poorly defined and is marked mainly by shallow earthquakes; toward the southwest and toward New Ireland BSSL seismicity increasingly overlaps with much larger subduction events, many exceeding magnitude 7, with southern New Ireland showing a particularly high concentration of such large earthquakes. Eastward plate geometry, especially within New Guinea, remains complex and is commonly constrained by the spatial distribution of shallow seismicity; structural entities such as New Guinea’s Finistree Block are routinely incorporated into regional models and are important for interpreting local tectonic behavior.