Earthquake Relocations Illuminate Tectonics along the Bismarck Sea Seismic Lineation, Papua New Guinea

The Bismarck Sea Seismic Lineation (BSSL) is a 1000-km-long band of shallow earthquakes that marks the boundaries between the North and South Bismarck Plates and the Manus Microplate, offshore Papua New Guinea. These tectonic boundaries comprise a series of transform faults separated by spreading ridges, and one extensional transform zone (ETZ). Since it was first identified in 1969, the deformation details of the faults along the BSSL have remained shrouded by the cloud of seismicity surrounding them. Here, we employ a recently published surface wave earthquake relocation algorithm (Howe etl al., 2019) to relocate events along the BSSL and the previously proposed Adelbert block boundary. These relocations show that left-lateral strike-slip seismicity concentrates narrowly along the known transform faults (the Schouten, Willaumez, Djaul, and Weitin) and the western ETZ. Several events that relocate to inside the Manus Microplate may represent distributed deformation by right-lateral bookshelf-style faulting. The spreading ridges are aseismic at our scale of observation, as previously suggested, but we identify clusters of strike-slip events at their terminations. The strike-slip events along the ETZ are at synthetic Riedel shear angles of \textasciitilde10° to those along the Willaumez transform. The earthquake relocations also show that the previously proposed SE boundary of the Adelbert block is not well defined or localized. We carefully selected the best quality relocated focal mechanisms to calculate new best-fit Euler poles for the transform segments. These calculations support earlier indications that the Schouten transform is not well fit by the same Euler pole as the other BSSL transforms, requiring internal deformation of the adjoining plates, for which we present additional evidence.