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The impact of the magnitude-7.3 temblor that hit Kumamoto Prefecture on April 16 was compounded by a small earthquake that struck shortly after in neighboring Oita Prefecture, seismologists say.
Scientists with the National Research Institute for Earth Science and Disaster Resilience said at an emergency meeting April 24 that the intensity of the April 16 quake registered around 6 on the Japanese scale of 7 in Oita Prefecture for this reason.
They said that if the earthquake at 1:25 a.m. in Kumamoto Prefecture, the most powerful so far in the series, had occurred in isolation, the intensity could have measured about 4 on the Japanese scale. That is strong enough to wake most people.
“The series of earthquakes (in Kumamoto Prefecture) is different (from others across Japan in recent years) in that they occurred in a volcanic region and induced many earthquakes,” said Shin Aoki, director of the institute’s Earthquake and Volcano Data Center, in Tsukuba, Ibaraki Prefecture. “An earthquake struck (in Oita Prefecture) while the impact of the earlier temblor (in Kumamoto Prefecture) was still being felt, which is rare.”
The institute’s seismic observation network registered the intensity of shaking from the April 16 earthquake at around 6 in Yufu, Oita Prefecture.
A detailed analysis of seismic waveforms in Yufu, where the institute’s observation point was set, showed that a bigger shock occurred about 16 seconds after seismic waves of the magnitude-7.3 temblor in Kumamoto Prefecture were first observed.
Similar seismic waveforms were also recorded in Kokonoe, Oita Prefecture.
The researchers said their analysis showed that a small earthquake occurred close to the vicinity of the observation point while the shock of the Kumamoto earthquake was still continuing.
In general, the farther a given location is from the epicenter of an earthquake, the less intense the vibration becomes.
But the shock of the April 16 quake registered around 6 in Oita Prefecture despite its distance from the epicenter of the magnitude-7.3 quake near the city of Kumamoto, according to the researchers.
Blocks of a shrine gate toppled in an earthquake in Yufu, Oita Prefecture, on April 16 (Hideki Yajima)
（ASAHI SHINBUM）
