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Questions and answers: The Kumamoto earthquakes

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Staff Writers

The series of huge earthquakes and aftershocks that have been rattling wide parts of Kumamoto and Oita prefectures since Thursday have raised fears that other regions in the nation might be struck by similar jolts in the near future.

Here are some questions and answers on seismic activity in Japan:

What type of earthquakes struck Kumamoto?

The 2016 Kumamoto Earthquake is actually a series of quakes that are being caused by two plates slipping against each other along an active inland fault. The events take place at a relatively shallow depth and cause the destruction of bedrock.

It is the same type as the Great Hanshin Earthquake of 1995 that hit Kobe and surrounding cities, killing over 6,000 people.

In contrast, the Great East Japan Earthquake that hit the Tohoku region in 2011, was caused by accumulated stress resulting from one tectonic plate being forced underneath another, resulting in what is called a “megathrust quake.”

What is unique about the 2016 Kumamoto Earthquake?

Whereas often a huge temblor hits first, followed by smaller aftershocks, a number of strong quakes have occurred following the first magnitude-6.5 quake on Thursday. The shaking has affected much wider areas than other quakes in the past, experts said.

The magnitude-7.3 quake that according to the Meteorological Agency was the main tremor struck the region 1½ days after the first one.

Why did we see such big quakes in relatively rapid succession?

Experts say the reason is not entirely known.

Of the 2,000 active faults around Japan, some 100 are designated by the government as key active faults. The Futagawa and Hinagu faults, along which the recent quakes occurred, are among the 100 most active and dangerous faults in the country.

The central government has conducted research on these 100 active faults over the past decade or so but was not able to predict the quakes that took place in Kumamoto, said Hiroyuki Fujiwara, a seismologist at National Research Institute for Earth Science and Disaster Prevention.

Are the focal points of quakes moving or expanding?

Fujiwara said the magnitude-7.3 quake on Saturday caught seismologists by surprise as they thought the initial quake — which turned out to be a precursor for Saturday’s — was an isolated tremor in a small section of the Futagawa fault.

Other quakes then took place further east. Some researchers say quakes may take place in succession along the lines of long faults, but no solid theory to explain such a scenario has been found, Fujiwara said.

Are these quakes precursors for others, especially along the Median Tectonic Line — the largest fault running from central Honshu to Kyushu?

Experts are not sure.

“We can explain what has happened, but it’s really hard to say what will happen,” Fujiwara said.

Takeshi Sagiya, a professor at Nagoya University’s Disaster Mitigation Research Center and an expert on crustal movement, said it is too early to worry about such a scenario.

Sagiya said he is more concerned about the southwestern side of the Hinagu fault in Kumamoto, where seismic activities appear to have been spreading in recent days.

A level-6 quake on the Japanese intensity scale of 7 may hit the fault in the near future, Sagiya said.

Is the small eruption of Mount Aso on Saturday related to the quake?

The view of volcanologists, as well as the Meteorological Agency, has been that the eruption was not triggered by the Kumamoto quakes, as its characteristics are no different from small-scale eruptions that have taken place before.

“There is probably no causal connection” between the earthquakes and the eruption, Chief Cabinet Secretary Yoshihide Suga told a news conference Saturday. “But we will keep monitoring (the volcano).”

Are the quakes in Kyushu and the magnitude-7.8 quake that hit Ecuador over the weekend — the largest since 1979 — related?

Fujiwara said they are not.

“The two locations are so far away from each other it’s impossible to suspect a link,” he said.

Are nuclear power plants in Kyushu safe?

Many citizens and anti-nuclear activists have expressed concern over the nuclear power facilities in Kyushu, in particular the two reactors running at the Sendai power plant in Kagoshima Prefecture, the only commercial nuclear plant now in operation in Japan.

The Nuclear Regulation Authority, however, has maintained that the Sendai plant does not need to be shut down because the strongest temblor registered at the plant since Thursday night was 8.6 gal (a unit used in seismology to express the acceleration of an earthquake), far lower than the safety level that would trigger an automatic reactor shutdown.

The criteria was set between 80 to 260 gal, depending on the direction of a shake and the strength of key components in the Sendai reactors.

All other reactors have been stopped in the wake of the 2011 Fukushima meltdown crisis, while power companies have applied for the NRA’s safety checks to restart many other reactors under the new safety standards drawn up after the Fukushima crisis.

At the Genkai nuclear power plant in Saga Prefecture, the strongest of the recent shakes was 20.3 gal. The reactors at the plant have long been shut down, but had they been active, they would be automatically shut down with a temblor of between 70 and 170 gal.

The Ikata nuclear power plant in Ehime Prefecture, which is also undergoing safety checks, is right by the Median Tectonic Line. The three reactors there have not shown abnormal activity since the quakes, according to Shikoku Electric Power Co. and the Ehime Prefectural Government.


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