Sitting atop two faults, Mashiki should expect more shocks

Kyodo

Strong aftershocks might rattle southwestern Japan in the coming days as a result of Thursday night’s magnitude-6.5 earthquake in central Kumamoto, the Meteorological Agency and seismology experts say.

As of 9 p.m. Friday, 147 aftershocks had been recorded since the quake struck at 9:26 p.m. Thursday, it said. The quake took the seven-tier Japanese intensity scale to its max, making it the first level 7 temblor to rock Kyushu.

By number of aftershocks logged within 12 hours of a quake, the Kumamoto temblor ranks third since 1995, the agency said. The top two spots are claimed by the Niigata earthquake of 2004, which resembles the Kumamoto quake, and the Iwate-Miyagi inland quake of 2008.

Thursday’s earthquake originated around 11 km below the town of Mashiki, an area of old houses where the Futagawa and Hinagu faults lie.

The agency said strong aftershocks often occur after shallow quakes because there is a tendency for the Earth’s crust to be weaker nearer the surface.

According to a government task force that researches earthquakes, there is a 0.9 percent probability of a 7.0-magnitude quake taking place on the Futagawa fault over the next 30 years, and a maximum probability of 6 percent of a 7.5-magnitude quake striking the Hinagu rift.

Despite heavy investment on research, the government has never succeeded in predicting a major earthquake.

The Futagawa fault stretches more than 64 km from the village of Minamiaso to the tip of the Uto Peninsula via Mashiki, while the Hinagu fault runs some 81 km from near Mashiki to the south of the Yatsushiro Sea, the task force says.

With rain forecasted for the area over the weekend, the area is at risk of landslides due to the quake-softened ground.

An upper 6 aftershock struck at three minutes past midnight and was followed by a lower 6. Two lower 5 shocks and 16 logged as 4s were also recorded.

In March 2013, the agency began measuring ground movement intensity for earthquakes of longer duration and came up with a four-level scale.

According to that scale, a Level 4 quake is too intense for people to stand. That intensity was recorded for the first time during Friday’s upper 6 aftershock.

“With many active faults still undiscovered, it would not be surprising for a similar quake to occur anywhere in the Japanese archipelago,” said Katsuhiko Ishibashi, professor emeritus of seismology at Kobe University.

“There is no guarantee that further shaking won’t be stronger than that of Thursday night, so rescuers and evacuees need to be extremely careful,” Ishibashi said.

The Meteorological Agency said it is unclear whether the temblor is connected to the faults. But Keiichi Tadokoro, associate professor at Nagoya University’s Earthquake and Volcano Research Center, strongly suspects a link.

“The largest intensity figure was recorded only in Mashiki, possibly because the ground there is easily shaken,” he said.

The damage is concentrated in a narrow area with many older houses that lack seismic reinforcement, reminiscent of the 1995 Kobe quake that killed about 6,500 people, said Yoshiteru Murosaki, head of the Education Center for Disaster Reduction at the University of Hyogo in Kobe.