A powerful earthquake struck northern Japan early Monday, intensifying concerns after a series of recent tremors and prompting warnings of more to come.

A magnitude 5.7 earthquake struck off the coast of Japan’s Hokkaido region early Thursday.

A magnitude 6.7 earthquake off Japan triggered a tsunami advisory for parts of Hokkaido and the coasts of Aomori, Iwate, and Miyagi prefectures on Friday.

The number of people injured in northeastern and northern Japan after a magnitude-7.5 earthquake struck the region has rises to 52.

An earthquake with a preliminary magnitude of 6.1 struck off the coast of Kushiro in Japan's Hokkaido on Saturday, local weather agency said.