The tremor struck around midday in the Pacific Ocean at a relatively shallow depth of 10 kilometers (6.2 miles). The U.S. Geological Survey measured a stronger 7.2 magnitude.
Japan and Russia both claim some of the sparsely populated islands in the remote region.
The epicenter was 58 kilometers (160 miles) east-northeast of Kuril'sk, Russia, and 528 kilometers (328 miles) northeast of Nemuro, Japan.
The nearest land is the volcanic islands of Urup, Iturup and Sumushir. Hokkaido officials said the islands were not under Japanese control.
The area is about 1,500 kilometers (930 miles) northeast of Tokyo.