An official from the Japan Meteorological Agency has urged people in the greater Tokyo area to stay vigilant for the next seven days or so against the possibility of more earthquakes as strong as Thursday's tremor.
Tsukada Shinya spoke at a news conference a few hours after a magnitude 5.9 quake shook Tokyo and surrounding areas on Thursday night. The focus of the quake was in Chiba Prefecture, east of Tokyo, at a depth of 75 kilometers.
Tsukada referred to a major quake that the central government says may hit directly beneath Tokyo. He said this possible temblor is expected to have a shallower focus and greater magnitude than Thursday's quake.
Tsukada said Thursday's quake occurred along a reverse fault after the earth's crust came under pressure from the east and west. He said that although more research is needed, the quake is thought to have originated in or near the border between the Philippine Sea Plate and the Pacific Plate.
Tsukada said whether the quake was a mainshock or a foreshock of a greater quake will remain unknown until scientists can observe the entire seismic activity.