Kyushu earthquakes’ shallow focus led to violent tremors
MASHIKI, Kumamoto (The Japan News/ANN) - Saturday’s magnitude-7.3 earthquake and Thursday’s magnitude-6.5 foreshock were both classic examples of shallow inland earthquakes with a focus directly below a populated area.
Saturday’s magnitude-7.3 earthquake and Thursday’s magnitude-6.5 foreshock were both classic examples of shallow inland earthquakes with a focus directly below a populated area.
Because both were shallow — the foreshock’s focus was 11 kilometres deep, while the main quake’s was 12 kilometres — the seismic waves they generated reached the surface without significantly weakening. Consequently, they violently jolted parts of Kumamoto Prefecture with a strength measuring 7 or upper 6 on the Japanese seismic intensity scale of 7.
Many homes and buildings collapsed in areas including the Kiyama district in the town of Mashiki in the prefecture. Near the local government office, walls and roof tiles were left scattered across roads by the force of the shaking. Even wooden homes that had improved their earthquake resistance, such as by using metal fittings to strengthen joints between pillars and crosspieces, had their bottom level crushed by an upper floor.
Fumio Yamazaki, a professor at Chiba University and an expert in earthquake disaster prevention engineering, examined the district after Saturday’s main quake.
“Even buildings that withstood the foreshock were destroyed or badly damaged by the main quake,” Yamazaki said. “I think that’s because the maximum intensity of the shaking was the same as that caused by the Great Hanshin Earthquake.”
The Great East Japan Earthquake of March 2011 was a trench-type quake, which occurs when plate boundaries under the sea rub against each other, generating tsunami and severe shaking over a wide area. Compared with this quake, the quakes that have battered Kyushu in recent days had a shallow focus under an inland area and its surrounding region, which tends to concentrate the damage in a narrower area.