The earthquake that struck off Fukushima Prefecture on Saturday caused violent tremors as strong as upper 6 on the Japanese seismic intensity scale of 7 in the Tohoku region, as well as milder shaking across wide areas stretching from Hokkaido to the Chugoku region.
There was no serious damage to buildings, or tsunami resulting in casualties. Damage is affected by the specific type of earthquake, and there is a risk of another major quake in the area.
A quake of upper 6 was recorded in Kunimi, Fukushima Prefecture. Ayumi Konno, a 35-year-old part-time employee in the town, said “the level of the tremor was about the same” as the Great East Japan Earthquake, which she also experienced 10 years ago.
This time, however, Konno could use tap water immediately after the quake, and electricity and gas were restored overnight.
In Zao, Miyagi Prefecture, which also saw an intensity of upper 6, no buildings collapsed from the Saturday quake. In contrast, 172 buildings were destroyed or damaged in the Great East Japan Earthquake, when the same upper 6 was recorded in the town.
“The damage was small in terms of the intensity level,” said Akira Chiba, crisis management supervisor of the prefectural government.
The strongest tremor recorded from the Saturday quake was 1,432 gals (a unit of acceleration that indicates the power of earthquakes), observed in Yamamoto, Miyagi Prefecture, according to the National Research Institute for Earth Science and Disaster Resilience. That is only slightly below the 1,580 gals recorded in Mashiki, Kumamoto Prefecture, in the foreshock of the 2016 Kumamoto earthquakes.
According to The Yomiuri Shimbun’s tally as of 5pm on Monday, about 1,700 buildings were damaged in Fukushima and Miyagi prefectures, but no buildings were confirmed to have been destroyed.
Strong seismic waves with a short period of less than one second (the time required for one full cycle of a wave) were detected in Soma, Fukushima Prefecture, and other areas where the maximum seismic intensity of upper 6 was observed, according to Yuki Sakai, a professor of earthquake disaster prevention engineering at Kyoto University.
Such waves, which produce small, rattling tremors, are readily felt by people and tend to damage small structures. This was the case in the 2018 earthquakes in northern Osaka Prefecture where a block wall collapsed, killing a girl. Roof tiles and block walls were damaged in Saturday’s quake as well.
Waves with a period of one to two seconds tend to cause the collapse of wooden houses and other structures, which was confirmed in the Great Hanshin Earthquake in 1995 and the Kumamoto earthquakes in 2016. This time, the waves with this period were relatively weak, which is believed to have resulted in little damage to buildings.
Still, Sakai warned: “Earthquakes that can destroy buildings can also occur in this area. I hope that people will take sufficient measures against earthquakes, instead of being overconfident that even an upper 6 quake was not a problem.”
The correlation between wave periods and the type of an earthquake is unclear. Researchers are examining the issue.