Washington - A strong 6.6-magnitude earthquake struck off the west coast of the US State of Alaska on Sunday, seismologists said, but no tsunami warning was issued.The US Geological Survey (USGS) said the quake struck at a depth of 111.8km and 60km east of the remote Buldir Island in the Aleutian archipelago that is home to some 8 000 people."Overall, the population in this region resides in structures that are resistant to earthquake shaking, though vulnerable structures exist," USGS said. "The predominant vulnerable building types are unreinforced brick masonry and reinforced masonry construction."The Pacific Tsunami Warning Centre did not immediately issue any warnings."There is a low likelihood of casualties and damage," USGS added, assigning the event a "Green alert".