The shaking rocked buildings in downtown Tokyo, awakening residents of the capital at 1:45 a.m. local time.
The Japan Meteorological Agency said the 6.7 magnitude earthquake was centered about 25 miles (40 km) beneath the Pacific off the coast of Ibaraki prefecture (state).
The agency said the six people who were injured were in Tokyo, Ibaraki, Saitama and Chiba prefectures, adding that the quake could also be strongly felt in Tochigi prefecture, north of Tokyo.
No significant damage was reported from the initial quake or any of the minor aftershocks that occurred during the following day.
