Seismic Event Location: Nonlinear Inversion Using a Neighbourhood Algorithm M. S. SAMBRIDGE 1 and B. L. N. KENNETT 1 Abstract Ð A recently developed direct search method for inversion, known as a neighbourhood algorithm (NA), is applied to the hypocentre location problem. Like some previous methods the algorithm uses randomised, or stochastic, sampling of a four-dimensional hypocentral parameter space, to search for solutions with acceptable data ®t. Considerable ¯exibility is allowed in the choice of mis®t measure. At each stage the hypocentral parameter space is partitioned into a series of convex polygons called Voronoi cells. Each cell surrounds a previously generated hypocentre for which the ®t to the data has been determined. As the algorithm proceeds new hypocentres are randomly generated in the neighbourhood of those hypocentres with smaller data mis®t. In this way all previous hypocentres guide the search, and the more promising regions of parameter space are preferentially sampled. The NA procedure makes use of just two tuning parameters. It is possible to choose their values so that the behaviour of the algorithm is similar to that of a contracting irregular grid in 4-D. This is the feature of the algorithm that we exploit for hypocentre location. In experiments with dierent events and data sources, the NA approach is able to achieve comparable or better levels of data ®t than a range of alternative methods; linearised least-squares, genetic algorithms, simulated annealing and a contracting grid scheme. Moreover, convergence was achieved with a substantially reduced number of travel-time/ slowness calculations compared with other nonlinear inversion techniques. Even when initial parameter bounds are very loose, the NA procedure produced robust convergence with acceptable levels of data ®t. Key words: Hypocentre, event location, neighbourhood algorithm. 1. Introduction The location of seismic events in space and time is an important part of seismological practice with a wide range of applications, but has particularly signi®cance in the context of monitoring a comprehensive test-ban treaty. A primary discriminant between man-made events and the majority of natural earthquakes is provided by the depth of the source. Drilling techniques for the emplacement of devices preclude a source depth of more than a few kilometres. Where the depth of an event can be established to be deeper than 15 km, the event is very unlikely to be 1 Research School of Earth Sciences, Institute of Advanced Studies, Australian National Univer- sity, Canberra ACT 0200, Australia. E-mail: malcolm@rses.anu.edu.au, brian@rses.anu.edu.au Pure appl. geophys. 158 (2001) 241±257 0033±4553/01/020241±17 $ 1.50+0.20/0 Ó Birkha È user Verlag, Basel, 2001 Pure and Applied Geophysics