The manner in which an earthquake produces a tsunami , or seismic sea-wave , is not well understood. In Japan and in Chile they accompany most large shallow submarine earthquakes close to the coast, while in New Zealand and much of the South West Pacific tsunamis of local origin have been of comparatively rare occurrence, and are usually small. Nevertheless, both tsunamis and seiches (resonant oscillations of enclosed bodies of water) constitute an appreciable component of our earthquake risk.