Studies warn of giant earthquakes hitting northern and southern Japan

Scientific research suggests that massive once-in-a-century earthquakes, which release huge amounts of energy and can trigger devastating tsunamis, are becoming increasingly likely in Japan.

The research shows that Hokkaido and Okinawa Prefectures are particularly at risk.

The studies combine new technological forms of research with the history of major seismic events across the country.

Historically, large earthquakes have been observed along the Chishima Trench, which runs south from the northeastern Pacific Ocean to eastern Hokkaido, and the Japan Trench, where the Pacific plate subducts off Hokkaido.

Tsunami impacts indicate that such destructive waves tend to occur in northern Japan at intervals of 340 to 380 years on average.

In 2017, the Government Headquarters for the Promotion of Earthquake Research declared an “extremely urgent situation” regarding the possibility of a major earthquake of magnitude 8.8 or stronger along the Chishima Trench.

This prediction was based on the fact that 400 years had passed since the last major tsunami occurred there during the first half of the 17th century.

A special warning was also issued after an earthquake off the northeastern Sanriku region in April this year for possible subsequent earthquakes.

Academic findings released in February confirmed the “extremely urgent need” in Hokkaido.

“The latest monitoring results confirm that the current situation requires a constantly high level of vigilance,” said Kazuyoshi Nanjo, a specially appointed professor of seismology at Shizuoka University.

Nanjo and his colleagues examined several small and medium-sized earthquakes to determine “B values” for specific regions.

Low b values ​​indicate the presence of energy accumulating underground with the potential to be released in a large earthquake.

Before the Great East Japan Earthquake of March 2011, the low b-value condition off the coast of Sanriku had persisted since the mid-2000s. But the b value increased significantly after the 9.0 magnitude earthquake occurred.

Scientists found a significantly lower b value off the coast of Hokkaido around 2008. The area is 20 square kilometres, smaller than the similar 100 km by 100 km site off Sanriku.

Both areas fall within “seismically free” zones where no major earthquakes have occurred for some time.

Slow earthquakes, or slow-moving landslides, and low seismic activity were also observed around the two sites.

In other words, the current situation off Hokkaido is similar to the situation off Sanriku shortly before the Great East Japan Earthquake.

“We cannot say that a massive earthquake will occur immediately and we cannot predict its size,” Nanjo said. “The only thing we can say is that people should be prepared for this to happen at any time so that disaster control efforts can be given the highest possible priority.”

Cortical motion analysis

A research team led by Fumiaki Tomita, an assistant professor of seismology at Tohoku University, set up equipment at three sites on the seafloor off Nemuru, Hokkaido, to monitor movements of the Earth’s crust. This was the first time such data had been obtained near an earthquake-free zone.

Tomita and his colleagues collected about five years of data starting in 2019, and found that both the subducting Pacific plate and the landward-dominant plate were moving at the same speed of 8 centimeters per year and in nearly identical directions.

The data indicate that tectonic plates are firmly fixed to each other. This would rule out the possibility that the plate in the seismically empty site is sliding too smoothly to produce large tremors.

The researchers’ conclusion was that the region would eventually reach its maximum limit, resulting in a massive earthquake.

During the last major earthquake, the displacement of plates in the area was estimated at 25 metres.

Based on the assumption that the plates have always moved since then as they do now, calculations show that enough stress has already built up to cause a displacement of 20 to 30 metres.

The area may already be in a state where another huge earthquake of the same size could occur.

“Energy builds up steadily, even though the intervals between earthquakes vary by several hundred years,” Tomita said. “We should expect earthquakes at any time.”

Supercycle earthquake off Okinawa

After the 9.1-magnitude Indian Ocean earthquake in 2004 and the Great East Japan Earthquake, scientists committed themselves to studying “supercycles,” that is, the repetition of giant earthquakes at intervals of several hundred years or longer.

These earthquakes release energy from a wide area at once as if they are swallowing up the central areas with the magnitude of 7 to 8 earthquakes occurring frequently.

In an article published in February, a team of scientists, including Mamoru Nakamura, a professor of seismology at the University of the Ryukyu, said there is a supercycle in the Ryukyu Trench, where the Philippine Sea plate is subducting.

The team examined fossilized coral colonies on land and in the sea southwest of Ishigakijima Island in Okinawa Prefecture.

Coral reefs grow near the ocean’s surface and retain traces of land uplifts associated with past earthquakes.

Team members reconstructed Earth’s crustal motions based on estimated scales and timings of uplifts. They compared the collected data with typical earthquake epicenters.

According to the study, two active periods with concentrated giant earthquakes have occurred over the past 5,000 years.

Major earthquakes frequently caused a rise of tens of centimeters at intervals of every 100 years, from 5,000 to 4,000 years ago, and also from 3,000 to 2,000 years ago.

Towards the end of periods of active seismicity, particularly large heights of 1 to 2 m are observed, likely caused by massive magnitude 9 earthquakes.

The newspaper indicates that the great Yaeyama tsunami that occurred in 1771 near Ishigakijima Island may have heralded the opening of the third supercycle.

The earthquake caused a tsunami 30 meters high, killing 12,000 people.

The height of the earthquake, estimated at about 23 cm, was smaller than its previous counterpart.

It was previously thought that the Ryukyu Trench was unlikely to experience a major earthquake, given the scarcity of large tremors in recent history.

However, this opinion seems to be changing.

“A period of frequent earthquakes may be approaching, although a few major earthquakes have been reported recently,” Nakamura said. “We have to realize that different types of earthquakes can occur.”

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