A powerful undersea earthquake has struck north of New Zealand, prompting a tsunami watch in the region. The US Geological Agency said the magnitude 7.7 quake hit Wednesday evening. It was centered at a depth of 10 kilometers (six miles) southeast of the Loyalty Islands, it said. It was not expected to cause significant damage or fatalities on land. The US Tsunami Warning Center issued tsunami watches for New Zealand, Fiji, Vanuatu and other Pacific islands. The region is prone to earthquakes because it sits along the Pacific `Ring of Fire,' a horseshoe-shaped series of seismic fault lines around the ocean.