The southwest quadrant of the Pacific, from southern New Zealand to central New Guinea, contains eight distinct active regions of the asymmetric type (i.e. continental margins and island arcs). These regions vary widely in size, shape, and geodynamic character. Four of them face towards and four away from the Pacific Ocean; hence the concept of a circum-Pacific active belt is only partly applicable. The area includes some of the most intensely active features known, such as the deep-focus seismicity of the Tonga region and the intermediate-focus seismicity of the New Hebrides region. There are none, however, of the great curved structures that are typical of the north and northwest Pacific areas. The southwest Pacific is a severe testing-ground for hypotheses concerned with large-scale processes and structures in the upper mantle and crust.