Use of two inverse techniques. Application to a local structure in the New Hebrides island arc

Two inversion techniques (stochastic inverse and linear programming) are used in the determination of hypocentre and velocity model parameters simultaneously by using P-wave arrivals from local earthquakes at a local seismograph array. These procedures are applied to P-wave data obtained from a combined land and ocean bottom seismographs network in the New Hebrides island arc. Two hundred and fifty-two residuals data are inverted simultaneously with 46 earthquake parameters. The displacements of each initial hypocentre location are practically the same with the two methods and are small (2–4km). The main result for the velocity model parameters is the existence at a low-velocity zone between depths of 10 and 19km and may be deeper. An estimate of the degree of confidence of the solution is determined and can confirm that the extremal values of the 7.3 km s-1 are 7.2 and 7.5 km s-1. These results may be related to an intrusion of ultrabasic magma from the mantle beneath Malekula and Efate Islands.