GEODETIC MEASUREMENTS OF CONVERGENCE ACROSS THE NEW-HEBRIDES SUBDUCTION ZONE

Between 1990 and 1994, geodetic measurements (GPS observations) have b een conducted across the New Hebrides subduction zone where the Austra lia 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 1 1.7 +/- 0.8 cm/yr. The rate at Efate is 10.3 +/- 0.9 cm/yr, oriented N 242 +/- 4. Both azimuths very well compare with slip vectors of the la st 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 rat es 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 Sant o where the plate coupling is very high, the very low convergence rate might be related to the absence of recent strong earthquakes. No sign ificant variations are detected for the baselines within the Australia plate.