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The plate boundary between the Pacific-Caroline and Australian plates in northwestern New Guinea is associated with a geographic concentration of earthquakes developed in the Ransiki region of the northeastern Bird's Head Peninsula (West Papua, northwestern New Guinea) at the intersection of the Ransiki and Yapen faults. We examine these earthquakes based on regional geomorphological and lithostratigraphical frameworks, field observations of surface ruptures and liquefaction phenomena, and focal mechanisms of historical earthquakes. The Ransiki earthquakes are a set of 29 earthquakes from the Global Centroid Moment Tensor catalogue in the period 1977–2019 (magnitudes of Mw4.9 to Mw7.5). In the east, focal mechanisms show sinistral movement along the east-west trending Yapen Fault including the Mw6.7 earthquake on 21 April 2012. The largest earthquake was on 10 October 2002 (Mw7.5) and along with other earthquakes mainly in the southwest were associated with dextral movement indicated by focal mechanism solutions on the northwest trending Ransiki Fault south of its intersection with the Yapen Fault. The southern part of the Ransiki Fault therefore indicates local north-northeast compression that is also evident in the newly recognised Wainoei Fault south of Yapen Island. The two largest earthquakes (10 October 2002, 21 April 2012) show ground effects associated with liquefaction, indicated by surface offsets, open fissures, and sand blows, that all occurred in saturated sediments of the Ransiki delta. Earthquakes in the Ransiki region show that west-southwest oblique plate convergence between the Australian and Pacific-Caroline plates is partitioned into east-west sinistral strike-slip motion along the Yapen Fault and north-northeast compression associated with the Ransiki Fault. [Display omitted] • Earthquakes are located at a fault intersection near the Pacific plate boundary. • Oblique-slip plate motion is partitioned into strike-slip and convergent parts. • Focal mechanism solutions indicate both sinistral and dextral movements. • Earthquake damage is mainly controlled by liquefaction in a Quaternary delta. [ABSTRACT FROM AUTHOR]

[Display omitted] • 230Th–U ages (2.7–246 ka) date emergent coral reefs in northwestern New Guinea. • Rapid uplift rates of up to 1.28 m/ka are indicated. • Rapid uplift reflects oblique subduction along the Manokwari Trough. • Late Cenozoic neptunian dykes are probably gravity related. New Guinea is a mountainous island in a tectonically active setting with regions experiencing rapid uplift rates as shown by the celebrated emergent coral reef terraces of the Huon Peninsula. In the northern Bird's Head Peninsula of northwestern New Guinea, rapid uplift has been a feature of the tectonic history over the past 2.7 Ma with the development of the mountainous and highly dissected Kemum High (up to 2200 m) associated with the formation of the Central Bird's Head Monocline, implying a long-term uplift rate approaching 0.8 m/ka. In this work, Late Quaternary uplift rates of up to 1.28 m/ka were determined for emergent coral reefs in the Manokwari region (northeast of the much larger Kemum High). The rapid uplift rates are based on 230Th-U ages (2.7 ± 0.3 to 246.3 ± 8.0 ka with most ages in the range 69–128 ka) on corals and coralline algae, sampled from calcarenites at elevations of 2.45–77.5 m above present sea level. The rapid uplift indicates neotectonic activity reflecting crustal thickening in the Quaternary and neptunian dykes probably related to gravitational instability. The present-day tectonic setting of the Bird's Head Peninsula is cryptic, but the combination of uplift and tectonic activity shown by the earthquakes are consistent with ongoing oblique convergence between the Bird's Head and the adjacent Pacific-Caroline plate. Over the past 2 Ma, plate motion between the Bird's Head Peninsula and the Pacific-Caroline Plate has been accommodated by incipient south-dipping subduction along the Manokwari Trough to the north (with longer-lived subduction along the New Guinea Trench) and sinistral strike-slip movement along the Yapen fault whereas the Sorong Fault has been largely inactive. [ABSTRACT FROM AUTHOR]

A major problem in the study of subduction complexes has been to distinguish between processes of mud diapirism, deformation of unlithified sediments, frontal accretion and underplating. In the Batemans Bay area, Cambro–Ordovician rocks of the south coast of New South Wales (eastern Australia) contain common mud-rich me´langes that occur amongst an imbricated succession of Early to Middle Ordovician quartz turbidites, Late Ordovician black mudstone and chert, Cambrian limestone and mafic volcanics. An early set of fluidal obliquely trending folds are restricted to the Late Ordovician black mudstone and chert and probably formed by gravitational down-slope movement on a lower trench slope. Features indicative of deformation of unlithified sediments are still preserved and include mud injections, delicate small-scale faulting and the absence of grain-scale deformation. Tight to isoclinal north–south folds with axial planar west-dipping cleavage overprint all units and formed during the younger main-phase deformation that post-dated early deformation in the prism. The me´lange has been strongly overprinted by this main-phase deformation. Early me´lange formation is attributed to mud diapirism involving intrusion along early accretionary faults, bedding planes and in pipe-like features resulting in chaotic outcrop and map patterns. [Copyright &y& Elsevier]