WASHINGTON - A powerful magnitude 7.5 quake struck off the Pacific island nation of Vanuatu, the US Geological Survey reported.It said the quake, which struck at 3:55 a.m. on Sunday local time, was centered 60 km southwest of the town of Port-Vila and was quite shallow, at a depth of 40.5 km.The Pacific Tsunami Warning Center said the major quake had not triggered a Pacific-wide tsunami but said: "Earthquakes of this size sometimes generate local tsunamis that can be destructive along coasts located within a hundred kilometers of the earthquake epicenter."