Seismic activity spreads southwest Japan
In the series of earthquakes focused in Kumamoto prefecture and elsewhere, strong seismic activity has been observed in Yatsushiro area and other places in the southwest of the prefecture where such activity had not been seen much before.
Toward the northeast, seismic activity has spread into Oita prefecture. “There is no precedent (in past observation results) of seismic activity occurring over such a wide range,” an official of the Japan Meteorological Agency said. “It has not shown any signs of stopping, so we recommend that people be on alert for strong tremors that may occur in the days to come.”
According to the agency, seismic activity first intensified in northeastern Kumamoto prefecture following the main magnitude-7.3 earthquake that hit Kumamoto city and surrounding areas at 1:25 a.m. Saturday. Quakes measuring lower 5 to upper 6 on the Japanese seismic intensity scale of 7 occurred several times in a zone stretching about 100 kilometers to the central part of Oita prefecture.
At 9:16 a.m. Saturday, a magnitude-4.5 earthquake occurred in Yatsushiro, Kumamoto prefecture, which is about 40 kilometers southwest of the focus of the main quake. Agency officials said small tremors increased in and around Yatsushiro after that.
The Hinagu fault zone lies in the southwest of the prefecture. It caused a magnitude-6.5 quake at 9:26 p.m. Thursday, which was a foreshock of Saturday’s main quake.
“The Hinagu fault zone may have moved further, due to effects of the main earthquake, and caused small-scale quakes,” said Prof. Shinji Toda of Tohoku University, who is an expert in earthquake geology.
The government’s Earthquake Research Committee, chaired by Prof. Naoshi Hirata of the Earthquake Research Institute of the University of Tokyo, held an urgent meeting Sunday and announced its opinion that the main earthquake on Saturday was caused by movements in parts of another fault zone called the Futagawa fault zone.
Regarding the possibility that other fault zones that have not moved yet will cause large earthquakes, Hirata said: “Today’s seismology doesn’t give us the knowledge to say. We have no idea.”
