Seismic experts believe a magnitude 7.3 quake that rocked southwestern Japan in the early hours of Saturday occurred on a separate fault zone from a massive temblor that struck barely a day earlier, and was triggered by a chain reaction involving a series of tremors.
The 1:25 a.m. earthquake, with the same intensity as the 1995 Great Hanshin Earthquake, was about 16 times as large as a magnitude 6.5 temblor that hit Kumamoto Prefecture Thursday night, the Japan Meteorological Agency said, describing the latest jolt as the main quake and the initial one as a foreshock.
The governmental Earthquake Research Committee concluded Friday that the magnitude 6.5 quake happened on the 81-kilometer-long Hinagu fault zone running along the central part of the island of Kyushu.
Shinji Toda, professor of earthquake geology at Tohoku University, said the 64-kilometer-long Futagawa fault zone located north of the Hinagu zone could have moved to cause the fresh quake.
The village of Minamiaso, where a major landslide occurred, and the city of Uto in Kumamoto, where the city hall was half destroyed, are located at either end of the Futagawa fault zone.
In the Kumamoto town of Mashiki, Takashi Nakata, professor emeritus of geomorphology at Hiroshima University, spotted a part of a fault exposed at the land surface. He said it is believed to have been caused by a slip on the Futagawa fault zone.
The agency also said there had been 252 quakes by 11 a.m. Saturday since the magnitude 6.5 temblor that occurred at 9:26 p.m. Thursday, with the number of such temblors hitting the second-highest since 1995.
Gen Aoki, head of the agency’s quake and tsunami monitoring department, said seismic activity has also been observed on the Beppu-Haneyama fault zone in Oita Prefecture, located east-northeast of the Futagawa zone, adding it may be related to a series of strong earthquakes in Kumamoto.
However, Takashi Azuma, a researcher of geomorphology at the National Institute of Advanced Industrial Science and Technology, warned monitoring of volcanic activity should be tightened.