Horizontal and vertical interseismic velocity fields in the Vanuatu subduction zone from GPS measurements: Evidence for a central Vanuatu locked zone

We analyzed interseismic velocity fields along the Vanuatu arc subduction zone from more than 45 GPS station measurements spanning 15 years. Convergence with respect to the Australian plate (AP) is normal to the trench, with amplitudes that vary greatly from a maximum of 170 mm a(-1) at Vanikoro to 27 mm a(-1) facing the D'Entrecasteaux Ridge (DER). Motions expressed relative to the Pacific plate (PP) highlight arc segmentation. Both the northern and southern segments undergo back-arc spreading in a roughly NE-SW direction, rotating anticlockwise and clockwise, respectively, while the central block moves eastward at a faster rate with respect to the PP than to the AP. More convergence is therefore accommodated at the eastern bound of the arc than at the Vanuatu trench. A right lateral strike-slip movement (26 +/- 1 mm a(-1)) along a zone normal to the trench (direction N70 degrees) is necessary to accommodate the rate variation between the central and southern parts of the arc. Horizontal and vertical rates indicate partitioning between Malekula-southernmost Santo and central-northern Santo blocks. Simple elastic modeling of the interseismic stage deformation gives locked zone characteristics (25 degrees dip and 50 km width) and a 54 mm a(-1) long-term convergence rate between the AP and the central part of the Vanuatu arc from GPS horizontal and vertical velocities estimated south of the DER. The modeling does not fit the observed velocities facing the DER, probably because it does not account for the inelastic deformation associated with buoyant ridge subduction and back-arc thrusting.