The Mw 7.5 November 24, 1999, Ambrym-Pentecost (Vanuatu) earthquake: preliminary data on seismicity, associated tsunami and crustal motions.

The November 26, 1999, Ambrym-Pentecost thrust-fault earthquake is the larg est known earthquake to have occurred in the back-are compressive belt of t he central part of the Vanuatu subduction zone. We report here the first da ta recorded by our local seismic and geodetic networks together with field observations collected on Pentecost and Ambrym in December 1999. The epicen tre was located at the northern tip of Ambrym island (16.11 degrees S, 168. 13 degrees E) at a depth of 18.75 km. Distribution of aftershocks recorded during the first nine hours already delineates a north-south trending, 100- km-long by 25-km-wide zone, from the south of Ambrym (16.5 degrees S) up to the northern part of Pentecost (15.5 degrees S), compatible with the magni tude of the main shock. The easternmost parr of Ambrym has been significant ly uplifted. A white fringe of dead red algae indicates that the coseismic uplift which is around Im at the eastern tip of the island vanishes rapidly westward. Re-observation of the West Ambrym GPS site indicates a 35-cm eas tward and 5-cm northward motion. These vertical and horizontal displacement s in Ambrym can be explained by a large slip motion (4.5 m) along a shallow (0-15 km) west-dipping thrust fault plane trending north-south and emergin g east of Ambrym and Pentecost islands. Tide gauge data on Efate (160 km aw ay from the epicentre) and field observations relative to the associated ts unami favour a landslide contribution instead of 3 purely seismic origin. ( C) 2000 Acadimie des sciences/Editions scientifiques el medicales Elsevier SAS.