GEODETIC MEASUREMENTS OF CONVERGENCE ACROSS THE NEW-HEBRIDES SUBDUCTION ZONE

Between 1990 and 1994, geodetic measurements (GPS observations) have been conducted across the New Hebrides subduction zone where the Australia plate subducts under the New Hebrides Arc. This paper establishes convergence rate variations along the trench. In the South, at Tanna, the relative motion is oriented N244 +/- 4 and has a uniform rate of 11.7 +/- 0.8 cm/yr. The rate at Efate is 10.3 +/- 0.9 cm/yr, oriented N242 +/- 4. Both azimuths very well compare with slip vectors of the last major earthquakes. In the North, the rate at Santo is only 3.6 +/- 1.2 cm/yr, oriented N253 +/- 26. The difference in the convergence rates between Santo on the one hand and Efate and Tanna in the other hand points to a right lateral shear zone between Santo and Efate. At Santo where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No significant variations are detected for the baselines within the Australia plate.