The Yomiuri Shimbun A chain of earthquakes like the ones that hit Kumamoto Prefecture and neighboring areas could occur in other regions, given the numerous active faults in the nation.
The Kumamoto Earthquake illustrated the fact that Japan is one of the most earthquake-prone countries. About 10 percent of the world’s earthquakes occur in the nation.
The Kumamoto Earthquake occurred in an active fault zone that was causing quakes following repetitive movements that created fissures in the ground.
Active faults move once every 1,000 years to tens of thousands of years, but it cannot be predicted exactly when.
There are 2,000 active faults across the nation, and 97 active fault zones that have been designated as key areas that could move within 30 years, the government’s Earthquake Research Committee announced previously.
The Hinagu fault zone caused a magnitude-6.5 quake on the night of April 14, and the Futagawa fault zone caused a magnitude-7.3 quake in the early morning of April 16. Both are designated as key fault zones. The first quake was followed only about 28 hours later by the second one.
“The quake on the night of April 14 triggered the main quake in the Futagawa fault zone,” said Prof. Kazuki Koketsu of the University of Tokyo’s Earthquake Research Institute.
“We have to be aware that if active faults are connected with others, a series of major earthquakes could occur in areas [other than Kumamoto Prefecture],” he said.
Earthquakes frequently occur in Japan because the country is located on four plates — out of a dozen or so that cover the surface of the Earth — that overlap each other.
Also, a sea plate is submerging beneath a land plate, causing tension to build up.
This is a major factor that causes active faults to move.
A new fault zone was found when the Iwate-Miyagi Inland Earthquake hit the Tohoku region in 2008. It had a magnitude of 7.2 and registered upper 6 on the Japanese seismic intensity scale of 7.
An earthquake may occur in the future in a fault zone that has not been found — just like the Iwate-Miyagi earthquake.
An earthquake with a focus just below an inland area like the Kumamoto Earthquake is likely to cause damage in limited areas compared to those that hit areas bordering seabed plates. It is consequently important for local governments to take disaster countermeasures.
Nobuo Fukuwa, director of Nagoya University’s Disaster Mitigation Research Center, said: “Areas that have not been hit by quakes recently should change their mind-set and [assume] that the time for a quake is approaching. They should prepare disaster countermeasures, led by local governments.”