The 2018 Mw 7.5 Highlands Earthquake in Papua New Guinea: Implications for Structural Style in an Active Fold and Thrust Belt

The Papuan Fold and Thrust Belt (PFTB) in Papua New Guinea is actively forming within a complex tectonic setting at the boundary of the obliquely converging Australian and Pacific plates. The tectonic setting and inaccessibility of the PFTB make it one of the least well-understood fold and thrust belts on Earth. On February 25, 2018, a M-w 7.5 earthquake occurred within the PFTB, triggering an aftershock sequence which included five events >= M-w 6. In this study, we combine seismological, GPS and remote sensing observations to investigate the spatiotemporal distribution of crustal deformation during these events. All earthquakes >= M-w 6 were related to reverse offset on northeast-dipping fault planes and five out of the six, including the mainshock, were associated with midcrustal focal depths (similar to 15-30 km). During the sequence, the PFTB underwent up to 1.2 m of uplift and ground deformation occurred over 7,500 km(2). Combining these observations with our geological knowledge of this convergent margin highlights the primary control of the northern Australian passive margin on PFTB structural style. We propose that the earthquake sequence was related to tectonic inversion on a hidden extensional fault system beneath the PFTB and lateral variations along this fault zone had significant influence on the complex distribution of ground deformation. It follows that this fault system has had an important control on the evolution of variable structural styles within the PFTB. This study highlights the complexity that can characterize the evolution and structural style of fold and thrust belts.