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Unusual mechanism of the Ambrym and Pentecost

An earthquake shook the South-West Pacific islands of Ambrym and Pentecost on 26 November 1999. It was the strongest thrust event ever recorded in central Vanuatu. Offshore and onshore data gathered by IRD researchers yielded clues as to the tectonic movements involved in the earthquake. A resulting model of the rupture mechanism showed that in this area of Vanuatu, slipping motions of the Australian oceanic plate under the Pacific plate are blocked to the West, which generates strong stresses to the East of the island arc (back arc). This situation makes the Ambrym earthquake a rare example of a back arc intraplate thrust event, occurring behind an oceanic subduction zone.

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).

Bénédicte Robert | EurekAlert!
Further information:
http://www.ird.fr/fr/actualites/fiches/2004/fiche193.htm

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