A shallow 6.4-magnitude quake struck off the Pacific nation of Tonga early Sunday, the U.S. Geological Survey said, but no tsunami warning was issued. The quake hit 345 kilometers (213 miles) northeast of the capital Nuku`alofa at a depth of 10 kilometers. The USGS PAGER (Prompt Assessment of Global Earthquakes for Response) issued a green alert level projecting a very low risk of casualties or economic losses. The Hawaii-based Pacific Tsunami Warning Center said there was no threat of a tsunami after the quake, which was followed by a 5.4-magnitude aftershock. Tonga, almost 2,000 kilometers (1,240 miles) northeast of New Zealand, lies on the so-called "Ring of Fire", where continental plates collide causing frequent seismic activity.