A 6.0-magnitude earthquake hit off the coast of the South Pacific island of Vanuatu on Monday. The US Geological Survey said the quake struck at a depth of just one kilometre, around 61 kilometres southwest of the capital Port Vila.A GeoScience Australia seismologist, said the quake was very shallow and produced a lot of surface water activity, but there was no tsunami warning issued. Vanuatu lies on the so-called Pacific Ring of Fire, a zone of frequent seismic activity caused by friction between shifting tectonic plates.