Those jolted by a late-night tremor are on the alert in Tokyo.
A magnitude 5.9 earthquake shook the capital area late Thursday, injuring more than 40 people.
Experts warn more seismic activity could be coming.
Tsukada Shinya of Japan Meteorological Agency said, "Based on past experience, a quake with a similar intensity occurred within about a week of a major earthquake in 10 to 20 percent of past cases. Please stay alert for the next week or so, if you are in an area where there was a strong tremor. Major quakes in particular tend to strike within a couple days after the first one."
The quake's focus was east of Tokyo, at a depth of 75 kilometers.
It hit at 10:41 p.m. Many on their way to bed --- felt a strong shake. In parts of Tokyo and Saitama prefecture, the quake registered an intensity of 5 plus on Japan's seismic scale of zero to seven.