Hakan Tanyas, Kevin Hill, Luke Mahoney, Islam Fadel, Luigi Lombardo 

Widespread landslide events provide rare but valuable opportunities to investigate the spatial and size distributions of landslides in relation to seismic, climatic, geological and morphological factors. This study presents a unique event inventory for the co-seismic landslides induced by the February 25, 2018 Mw 7.5 Papua New Guinea earthquake as well as its post-seismic counterparts including the landslides triggered by either aftershocks or succeeding rainfall events that occurred between February 26 and March 19. We mapped approximately 11,500 landslides of which more than 10,000 were triggered by the mainshock with a total failed planimetric area of about 145 km2. Such a large area makes this inventory the world’s second-largest recorded landslide event after the 2008 Wenchuan earthquake. Large landslides are abundant throughout the study area located within the remote Papua New Guinea Highlands. Specifically, more than half of the landslide population is larger than 50,000 m2 and overall, post-seismic landslides are even larger than their co-seismic counterparts. Our analyses indicate that large and widespread landslides were triggered as a result of the compound effects of the strong seismicity, complex geology, steep topography and high rainfall. We statistically show that the 15-day antecedent precipitation, as a predisposing factor, contributes to the spatial distribution of co-seismic landslides. Also, we statistically demonstrate that the cumulative effect of aftershocks is the main factor disturbing steep hillslopes and causing the initiation of very large landslides up to the size of ~5 km2. Taking aside the role of the intense seismic swarm and antecedent precipitation, these inventories also provide evidence for landslide events where the active tectonics contribute to weaken hillslopes and the fatigue damage. Overall, the dataset and the findings provided by this paper is a step forward in seismic landslide hazard assessment of the entire Papua New Guinea mainland.