SINGAPORE (Reuters) -The 8.8 magnitude quake off Russia that triggered tsunami warnings across the Pacific occurred on what is known as a "megathrust fault", where the denser Pacific Plate is sliding underneath the lighter North American Plate, scientists said. The Pacific Plate has been on the move, making the Kamchatka Peninsula area off Russia's Far East coast where it struck especially vulnerable to such tremors - and bigger aftershocks cannot be ruled out, they said.