The powerful earthquakes that struck Kumamoto Prefecture over the past three days have shocked the nation and renewed the public’s fear of natural disasters.
But some seismologists were not surprised at all, saying evidence suggested the writing was on the wall, although they didn’t know exactly when it would happen.
On Thursday, a magnitude-6.5 quake hit the maximum of 7 on the seven-level Japanese intensity scale, taking the nation by surprise. This was followed by Saturday’s magnitude-7.3 quake, which registered as an upper 6.
But Nagoya University professor Yasuhiro Suzuki said a magnitude-6.5 earthquake is relatively small for the Futagawa-Hinagu fault link, which has caused hundreds of quakes since Thursday night.
Suzuki said that given the size of the fault, experts had been expecting even larger quakes.
“That fault is so big experts have said it could trigger a magnitude-8 earthquake,” Suzuki told The Japan Times on Saturday.
“I was concerned that the magnitude-6.5 one wasn’t the end of the disaster because its size was relatively small,” he said.
Saturday’s quake hit near the Futagawa fault, which is linked to the Hinagu fault. Together, the two comprise a massive 101-km fault line.
Suzuki warned that more quakes were likely to keep rattling Kumamoto and surrounding areas, although hundreds of aftershocks have already taken place, striking terror into the hearts of millions of residents.
“Some parts of the fault were affected, but more earthquakes could happen because other parts did not move this time,” he said.
As of 11 a.m. Saturday, at least 252 quakes have been logged since the magnitude-6.5 earthquake hit at 9:26 p.m. Thursday, according to the Meteorological Agency.
Hideki Shimamura, a professor at Musashino Gakuin University, said the shallow depths of the epicenters of the quakes and diverging fault lines are the reasons for the frequent shaking. The epicenters for the magnitude 6.5 quake on Thursday was 11 kilometers below ground and 12 kilometers for the magnitude-7.3 temblor on Saturday.
“The same thing happened during the 2004 Niigata Chuetsu earthquake in which the epicenter was also shallow,” said Shimamura. “If the epicenter is about 100 kilometers, there won’t be so many aftershocks.”
Technically speaking, quakes are occurring not only in the “Kumamoto region” in southern Kumamoto Prefecture, but also in the Aso region in Kumamoto’s north, and in Oita Prefecture.
The agency has concluded the Kumamoto temblors are different from the ones in Aso and Oita, adding it has never seen a series of quakes occur over such a wide region.
Early Saturday, the agency said Thursday’s earthquake was a “foreshock” that prefaced Saturday’s quake, which was about 16 times bigger.
Saturday’s earthquake had the same intensity as the 1995 Great Hanshin Earthquake, which reached level 7 on the Japanese scale, and is now considered the “main shock.”
The government’s Earthquake Research Committee said Friday that Thursday’s quake happened in the Hinagu fault zone running along central Kyushu.
Meanwhile, Manabu Takahashi, a professor at Ritsumeikan University, pointed out that the string of quakes could be a precursor to more in other regions.
Mount Aso, which the agency said had a small eruption after Saturday’s quake, sits on the Japan Median Tectonic Line, the longest fault system in Japan, which runs from Nagano Prefecture to the Kyushu region.
The line runs through Nagano, Shizuoka, Aichi, and Wakayama prefectures, as well as the Shikoku region and Ise Shima in Mie Prefecture, where the Group of Seven summit will be held next month.
“In the worst case scenario, it’s possible that there are going to be similar quakes in those areas, including Nankai megathrust earthquakes,” said Takahashi.