Geographical and geological relationships of the New Hebrides Trench in the central and southern Vanuatu subduction zone. Subduction is towards the north east and this map does not show the North New Hebrides (Torres) Trench. Zealandia is outlined in black.
Approximate surface projection on Pacific Ocean of New Hebrides Trench (dark blue).
The trench was first described in 1962 by the U.S. research vessel "Spencer F. Baird", in the Scripps Institution of Oceanography Proa Expedition[2] and was explored in more detail in 2013 by the University of Aberdeen's Oceanlab team. They found cusk-eels, prawns, and other eels, and crustaceans. This is significantly different from other deep sea trenches that have been studied.[4]
At the New Hebrides Trench, the Australian plate is being subducted underneath the New Hebrides microplate in the Vanuatu subduction zone (previously called the New Hebrides subduction zone) towards the east where the trench has a north-south orientation. The trench is to the northeast of the Zealandia continental margin. The convergence rate in the subduction zone ranges from 120 mm (4.7 in)/yr in the south, to 40 mm (1.6 in)/yr in the central section, to the high rate of 170 mm (6.7 in)/yr in the north at about latitude 11°S in the Solomon Island region north of the Torres.[5] The anomalous lack of convergence in the central section is caused by the subduction of the d'Entrecasteaux Ridge. The progressive subduction/collision of the NW–SE trending Loyalty Ridge located on the Australian plate under the southern Vanuatu microplates produces much earthquake activity[5] but the most southernmost part of the trench south of latitude 22.5° S and east of longitude 170° E is not as highly tectonically active[6] It has been suggested that the current northern subduction to the south of the bend to the east in the trench should be considered as a separate subduction zone, called the Matthew and Hunter subduction system or subduction zone given its immature current volcanic arc and other characteristics.[7][8] It translates into the non subducting Hunter Fracture Zone which is a transform faulting fracture zone.[9] From 3 million years ago the southernmost Central Spreading Ridge of the North Fiji Basin propagated southward and has now intersected with the New Hebrides Trench and the Hunter Fracture Zone to form a triple junction with the Conway Reef Plate.[9]
Associated seismicity
The area of the southern part of the subduction zone between the latitudes 21.5 and 22.5° S and the longitudes 169 and 170° E is very active.[5] There have been multiple earthquakes including swarms of magnitude Mw 7.0+ during recent decades impacting on New Caledonia and Vanuatu.[5] The strain accumulation is regularly partially released through moderate to strong earthquakes during sequences which have included both interplate thrust faulting earthquakes and outer rise normal faulting earthquakes west and south-west of the trench.[5]
The Mw 7.7 2021 Loyalty Islands earthquake (Matthew Island earthquake) was much stronger than the usual seismicity on the southernmost aspects of the trench.[10] The epicenter was close to Matthew Island, and was both preceded and followed by a seismic crisis of multiple events with greater than Mw 5.0[10]
Major earthquakes associated with the subduction zone
The region could trigger tsunamis with a main propagation axis striking from WSW–ENE (putting New Caledonia and south Vanuatu most at risk) to S–N (putting northern New Zealand and Vanuatu most at risk).[5] The 5 December 2018 Mw 7.5 normal faulting earthquake generated a tsunami of more than 2 m (6 ft 7 in) in southern New Caledonia and Vanuatu.[5]
^ abcIoualalen, Mansour (May 2017). "Investigating the March 28th 1875 and the September 20th 1920 earthquakes/tsunamis of the Southern Vanuatu arc, offshore Loyalty Islands, New Caledonia". Tectonophysics. 709: 20. Bibcode:2017Tectp.709...20I. doi:10.1016/j.tecto.2017.05.006.