Kumamoto quake traced to fault that struck same region 10 years ago, with more tremors possible

A truck lies damaged on a road near Aeon Mall Kumamoto in Kashima, Kumamoto Prefecture, after a magnitude-7.1 earthquake struck near Uki on Tuesday. [AFP]

The magnitude-7.1 earthquake that struck near Uki in Kumamoto Prefecture, western Kyushu, on Tuesday was likely caused by the reactivation of the same fault believed to have triggered a powerful quake in the same region a decade ago, experts say.

Seismologists cited by the Asahi Shimbun and Yomiuri Shimbun on Wednesday said the Tuesday quake is thought to have originated from activity along the Hinagu fault zone — the same fault system that caused a magnitude-6.5 earthquake on April 14, 2016.

The Hinagu fault zone runs roughly 81 kilometers in a northeast-to-southwest direction through mountains stretching from Mashiki town in Kumamoto Prefecture to the Yatsushiro Sea to the west.

Japan's Earthquake Research Committee warned earlier this year that the Hinagu fault's inland section could produce a magnitude-7.5 quake, while its Yatsushiro Sea section could generate one of around magnitude 7.3 — and that full activation of the entire fault zone could trigger a magnitude-8.0 earthquake. The committee estimated the probability of a quake occurring in the area within 30 years at up to 6 percent, which the Yomiuri said corresponds to the S rating — the highest seismic risk classification among active faults across Japan.

Earthquakes caused by active faults tend to originate at relatively shallow depths, producing intense surface shaking that can cause widespread damage including building collapses and landslides. The focal depth of Tuesday's quake was also relatively shallow, at 10 kilometers.

Aeon Mall Kumamoto in Kashima, Kumamoto Prefecture, is seen after a magnitude-7.1 earthquake struck near Uki on Tuesday. [AFP]

During the 2016 Kumamoto earthquakes, a magnitude-6.5 quake originating from the Hinagu fault was followed two days later by a magnitude-7.3 tremor, leaving more than 270 people dead. The second, stronger quake was attributed to the Futagawa fault, which runs from the mountainous interior of central Kumamoto Prefecture westward toward the sea and partially overlaps with the Hinagu fault and a fault zone to the northeast.

Nao Hirata, chair of Japan's Earthquake Research Committee, urged the public to remain on full alert for strong shaking comparable to Tuesday's quake in the days immediately ahead. "As the 2016 Kumamoto earthquake showed, we must be fully prepared for powerful shaking of similar intensity in the coming days," he said.

Kimiyuki Asano, a professor at Kyoto University who investigated the 2024 Noto Peninsula earthquake, said authorities need to examine fault movement from both the 2016 Kumamoto earthquake and the current quake to determine whether activity along the Hinagu fault zone has run its course or has only just begun.

Japan's National Institute of Advanced Industrial Science and Technology had previously analyzed that when stress along the northern section of the Hinagu fault zone was released in 2016, the southern section became more susceptible to future seismic activity — and the area near Uki, where Tuesday's quake struck, falls within that zone.

Japan's Meteorological Agency warned that, just as the 2016 Kumamoto sequence produced back-to-back quakes exceeding magnitude 7 within two days, an even larger earthquake could follow the initial tremor this time as well.

When the magnitude-6.5 quake struck during the 2016 Kumamoto sequence, authorities warned the public to watch for aftershocks — but a stronger magnitude-7.3 quake followed two days later, prompting criticism that the term "aftershock" had reduced public vigilance and left people less prepared for further damage.

A road near Aeon Mall Kumamoto in Kashima, Kumamoto Prefecture, is seen after a magnitude-7.1 earthquake struck near Uki on Tuesday. [AFP]

Tuesday's earthquake registered a seismic intensity of 7 on Japan's scale — the first time that level has been recorded in the country since the January 2024 Noto Peninsula earthquake, which killed more than 700 people.

It is only the eighth time Japan has recorded a seismic intensity of 7 since the 1995 Great Hanshin Earthquake, which killed 6,434 people. Three of those eight occurrences have been in Kumamoto Prefecture alone. The latest quake also brought fresh damage to areas still bearing the scars of the 2016 disaster — including a section of Kumamoto Castle's stone walls that had collapsed in the earlier quake and was still under repair, which crumbled again.

Meanwhile, TV Asahi reported that Japan's Geospatial Information Authority observed a maximum crustal displacement of 84 centimeters in Yatsushiro following Tuesday's quake.

yckim6452@heraldcorp.com