Unusual mechanism of the Ambrym and Pentecost Islands earthquake in Vanuatu

The Vanuatu island arc, in the South-West Pacific, is 1 700 km long. It corresponds to a convergence zone where the Australian plate is slipping eastwards under the North Fiji Basin, which is part of the Pacific plate, thus generating earthquakes. On 26 November 1999, the central islands of Vanuatu, particularly Ambrym and Pentecost, were strongly shaken by a 7.5 magnitude surface earthquake followed by a tsunami. The earthquake and the many landslips it generated caused 10 deaths and considerable damage.

Immediately after the earthquake, IRD researchers conducted onshore and offshore investigations in order to unravel the tectonic movements and the rupture mechanisms brought into play. The eastern end of Ambrym was uplifted by more than a metre, whereas Pentecost to the North and Paama and Lopévi Islands to the South were not. Such vertical movement was indicated by biological markers in the form of the death of coral colonies and, along the coastline, by the appearance of a white band resulting from desiccation of encrusting red algae. Seismic data revealed the focus to be at the northern point of Ambrym, at about 15 km depth. The amount of uplift observed decreases rapidly towards the West and falls away to nothing a few kilometres from the eastern point of the island. This decrease confirms that the earthquake epicentre was nearby and fairly close to the surface. Moreover, measurement of the co-seismic horizontal movements produced by the earthquake at the different GPS network sites deployed over all the central islands showed that the west point of Ambrym was thrust 35 cm towards the East.

Combined data on aftershock distribution and vertical and horizontal motion revealed a strong thrust movement, with average slip of 6.5 m, of the North Fijian Basin crust under the New Hebrides arc that occurred along a West-dipping North-South surface fault emerging East of Ambrym and Pentecost. Ocean floor mapping along the eastern edges of Ambrym and Pentecost indicated a large fault scarp – 400 m wide, 40 km long and 900 m high – oriented 165° N. This scarp appears to correspond to the surface emergence of a rupture zone involved in the earthquake of 26 November 1999 (1).

The uplift observed at the eastern point of Ambrym appears to fall into a long-term process, begun between a few thousand and several million years B.P. The boundary of the uplift zone coincides with the point on the south-east coast of Ambrym where the fringe lagoon has disappeared. Moreover, the discovery of coral banks at heights of 2 to 10 m on the stretch of coast that suffered this co-seismic uplift suggests that the same thrust fault is reactivated regularly, tracing in this way the East coast of Ambrym and increasing the size of the scarp situated East of the island. Dating of the coral banks was conducted by measuring the Uranium/Thorium ratio present in the corals. From it an estimate was made of an average uplift rate of 3–4 mm per year over the course of the past 8000 years. The research team thus reckoned that the thrust fault scarp East of Ambrym resulted from activity dating back to 15 000 to 60 000 years. The return period of strong earthquakes has been calculated at between 100 and 375 years.

The November 1999 earthquake and the seismic cycle along the zone East of the island arc therefore show the plate to have undergone thrust deformation. Convergence produces thickening of the back-arc crust and continual generation of relief. This is how the islands of Maewo and Pentecost came into being. The thrust zone involved in the central Vanuatu earthquake therefore appears to be an active thrust front, which gradually incorporates fragments of lithosphere from the North Fiji Basin. This situation constitutes a rare example of a back-arc intraplate thrust event involving a thrust front behind an oceanic subduction zone.

Mina Vilayleck – DIC
Translation : Nicholas Flay

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