The frequency of the earthquakes that have hit Kumamoto Prefecture and surrounding areas is the highest ever observed in Japan for inland or coastal temblors, the Japan Meteorological Agency (JMA) said on April 17.
Moreover, the JMA said the seismic activity is spreading not only northeast toward Oita Prefecture, but also southwest.
"The seismic activity remains intense. Be careful of strong vibrations as well as rain," which could trigger landslides and mudslides, said Gen Aoki, director of the JMA's Earthquake and Tsunami Observation Division.
According to the agency, the number of earthquakes of at least magnitude-3.5 in Kumamoto and surrounding areas since the first M-6.5 temblor struck on the night of April 14 had hit 162 by 8:30 a.m. on April 17. The figure exceeded that for temblors following the M-6.8 Chuetsu earthquake in central Niigata Prefecture in 2004, which had held the previous high. The agency began observing the frequency of earthquakes measuring at least M-3.5 in 1995.
The number of quakes has increased since the M-7.3 main quake in the early hours of April 16, JMA officials said.