Quadripartite Surface Wave Method: Development

In general, accurate surface wave phase velocities are obtained from the tripartite method only when the epicentre-to-network path is nearly parallel to one leg of the array. This limitation can be removed by adding a fourth station to the network and assuming wavefronts in the form of circular arcs. The addition of two stations permits the use of such wavefronts to estimate the lateral heterogeneity of the structure beneath the array. This can also be done with a five-station network if two separate events are employed. Tests of the quadripartite method in the presence of lateral heterogeneity, show that a roughly rectangular configuration of stations yields a phase velocity which is a good approximation to the true velocity at the centre of the array. Tests of a triangular network which includes a central station indicate that such an array may provide the means for determining the phase-velocity gradient with only four stations.