The Mw 7.5 1999 Ambrym earthquake, Vanuatu: A back arc intraplate thrust event

Abstract

On 26 November 1999, a Mw 7.5 thrust earthquake occurred in the Vanuatu back are seismic zone. The aftershocks seismicity defines a west dipping plane down to a depth of 20 km consistent with the focal mechanism geometry. The shallow focal depth of the main shock and the coincidence of the surface projection of the plane of aftershocks with a 600 m high sea bottom fault scarp indicate the fault plane ruptured up to the surface. The size of the fault has been estimated to 50 × 25 km2 from the aftershocks distribution and modeling of coseismic vertical motion inferred from biological markers. The inferred length indicates the rupture affected the whole Ambrym segment, but inversion of GPS horizontal displacement data indicates most of the seismic moment has been released on a smaller 35 × 20 km2 fault surface with an average slip of 6.5 m. Foreshock and aftershock seismicity are organized in planar seismic zones parallel to the inferred main shock rupture plane. This spatiotemporal distribution of imaged fault suggests that the thrust front propagates in times by eastward jumps to new intraplate faults occurring in the western edge of the North Fiji Basin. The back arc present setting could eventually represent the very early stages of an incipient subduction zone with reverse polarity. The intraplate fault model yields shortening by thickening of the back arc crust. Using crustal balanced sections to model the deformations over the last 1.8 m.y. period, a maximum shortening of 55 km has been found.