Once-in-centuries megaquakes that release enormous amounts of energy and can spawn devastating tsunami are becoming increasingly likely in Japan, according to scientific papers.
At particularly high risk are Hokkaido and Okinawa Prefecture, the research shows.
The studies combine new technological forms of research and the history of major seismic events around the country.
Large earthquakes have historically been observed along the Chishima Trench, which extends south from the northeast Pacific Ocean to east of Hokkaido, and the Japan Trench, where the Pacific Plate subducts off Hokkaido.
Traces of tsunami indicate that such destructive waves tend to occur in northern Japan at intervals of 340 to 380 years on average.
In 2017, the government’s Headquarters for Earthquake Research Promotion announced a “high urgency” concerning the likelihood of a major earthquake with a magnitude of 8.8 or stronger along the Chishima Trench.
This projection was based on the fact that 400 years had passed since the last significant tsunami occurred there during the first half of the 17th century.
A special warning was also issued after an earthquake struck off the northeastern Sanriku region in April this year for potential subsequent quakes.
Academic findings released in February corroborated the “high urgency” in Hokkaido.
“The latest observation outcomes confirm that the current situation requires a high level of vigilance on a continual basis,” said Kazuyoshi Nanjo, a specially appointed professor of seismology at the University of Shizuoka.
Nanjo and his colleagues examined numerous small- to medium-sized earthquakes to determine “b-values” of certain areas.
Low b-values suggest accumulated underground energy with the potential to be released in a megaquake.
Before the Great East Japan Earthquake in March 2011, a low b-value state had persisted off the Sanriku coast since the mid-2000s. But the b-value rose dramatically after the magnitude 9.0 temblor struck.
The scientists found a significantly low b-value off the coast of Hokkaido around 2008. The area is 20 kilometers square, smaller than the similar 100-kilometer-by-100-kilometer location off Sanriku.
Both areas lie within “seismically blank” regions where major quakes had not occurred for some time.
Slow earthquakes, or slow-moving ground slips, and decreased seismic activity were also observed around the two locations.
In other words, the current situation off Hokkaido resembles that off Sanriku shortly before the Great East Japan Earthquake.
“We can neither say that a megaquake will happen immediately nor predict its scale,” Nanjo said. “The only thing we can say is that people should prepare for it to occur at any time so that anti-disaster efforts can be given the highest possible priority.”
CRUSTAL MOVEMENT ANALYSIS
A research team led by Fumiaki Tomita, an assistant professor of seismology at Tohoku University, set up equipment at three sites on the seabed off Nemuro, Hokkaido, to monitor crustal movements. It was the first time such data has been obtained near a seismically blank area.
Tomita and his colleagues gathered about five years of data beginning in 2019 and found that both the subducting Pacific Plate and the overriding plate on the landward side were moving at the same speed of 8 centimeters per year and in nearly identical directions.
The data indicated the tectonic plates are firmly fixed with each other. This would rule out the possibility that a plate in the seismically blank location is subducting too smoothly to trigger significant temblors.
The researchers’ conclusion was that the area will eventually reach its limit, leading to a massive earthquake.
During the last megaquake, the plate displacement in the area was estimated at 25 meters.
Based on the assumption that the plates have since always moved as they do now, calculations show that sufficient strain has already accumulated to cause a displacement of 20 to 30 meters.
The area may already be in a state where another megaquake of the same scale could happen.
“Energy has steadily been piling up, although the intervals between earthquakes vary by several hundred years,” Tomita said. “We must expect that temblors could occur at any time.”
QUAKE SUPER-CYCLE OFF OKINAWA
After the magnitude 9.1 Indian Ocean earthquake in 2004 and the Great East Japan Earthquake, scientists committed themselves to study “super-cycles,” the recurrence of megaquakes at intervals of several hundred years or longer.
Those quakes release energy from a wide area all at once as if they swallow the epicentral areas of 7 to 8 magnitude quakes that occur more frequently.
In an article published in February, a team of scientists, including Mamoru Nakamura, a seismological professor at the University of the Ryukyus, said a super-cycle exists in the Ryukyu Trench, where the Philippine Sea Plate subducts.
The team examined fossilized coral colonies on land and in the sea southwest of Ishigakijima island in Okinawa Prefecture.
Corals grow close to the ocean surface and preserve traces of ground uplifts linked to past earthquakes.
Team members reconstructed crustal movements based on the estimated scales and timings of the uplifts. They compared the collected data with model epicenters.
According to the study, a pair of active periods with concentrated megaquakes occurred over the past 5,000 years.
Major earthquakes repeatedly caused uplifts of dozens of centimeters at intervals of 100 years between 5,000 and 4,000 years ago, and also between 3,000 and 2,000 years ago.
Toward the end of the active quake periods, especially large uplifts of 1 to 2 meters were observed, likely resulting from megaquakes of 9-level magnitudes.
The paper points out that the 1771 Great Yaeyama Tsunami near Ishigakijima island may have heralded the opening of the third super-cycle.
The quake-triggered a 30-meter high tsunami that killed 12,000 people.
The quake’s uplift, estimated at 23 cm, was smaller than that of its earlier counterparts.
It had previously been believed that the Ryukyu Trench was unlikely to experience a major earthquake, given its scarcity of large tremors in recent history.
However, that view appears to be changing.
“A period of frequent earthquakes may be approaching, even though few major quakes have been reported lately,” Nakamura said. “We need to recognize that various types of earthquakes can occur.”
AloJapan.com