WASHINGTON-A strong but deep 7.7-magnitude earthquake struck off the Northern Mariana Islands in the western Pacific on Saturday, but triggered no tsunami alert, US monitors said. The quake struck at a depth of 132 miles (212 kilometers) at 7:18 am local time (2118 GMT Friday) some 19 miles southwest of the island of Agrihan, according to the US Geological Survey. No tsunami warning was issued. A self-governing commonwealth of the United States since 1976, the Northern Marianas consist of 15 islands, with more than 90 percent of the population of 54,000 living on the main island, Saipan. Only four other islands are populated.