A record 262 aftershocks of magnitude 5 or greater were registered in the seven days following the 9.0-magnitude Tohoku temblor on March 11, according to the Meteorological Agency.
The frequency of aftershocks of that magnitude until noon Friday was the highest recorded in Japan and more than 2.5 times the frequency detected after a magnitude 8.2 earthquake struck eastern Hokkaido in 1994.
Meanwhile, the agency lowered the likelihood of aftershocks measuring at least upper 5 on the Japanese seismic intensity scale of 7 within three days to 30 percent from 40 percent.
There have been three aftershocks of magnitude 7 or greater since the initial quake, including a magnitude 7.5 temblor that occurred shortly afterward, while there have been 49 aftershocks of magnitude 6 or more.
The aftershocks have occurred in waters off a wide stretch of land from Iwate to Ibaraki prefectures in a zone 500 km long and about 200 km wide.
"There has been a slight downward trend, but they remain frequent," said Takashi Yokota, head of the agency's Earthquake Prediction Information Division. "We need to remain vigilant because an earthquake focused in an oceanic area could cause strong aftershocks as late as 10 to 20 days afterward."
Seismic waves fast
BEIJING (Kyodo) Seismic waves reached North Korea "three to four minutes" after a magnitude 9.0 quake rocked northeastern Japan last week, a monitoring group reported, quoting a North Korean seismologist.
When the waves reached North Korea, many of the ground water observatories saw a fall in water levels, the seismologist of the Earthquake Bureau was quoted as saying.