Expert Sounds Alarm on Unknown Megathrust Earthquake in Kanto Region

September 1 marks the 100th anniversary of the devastating Great Kanto Earthquake. Recent studies suggest another megathrust earthquake of an unknown type could occur in the Kanto region. However, these findings are still not widely known. 

If this earthquake occurs, it is also expected to cause a giant tsunami. Experts are calling for hastening preparations based on the lessons learned from the Great East Japan Earthquake.

In Sagami Bay, off the coast of Kanagawa Prefecture, a submarine trench called the Sagami Trough extends from east to west. Here, the Philippine Sea Plate is subducted beneath the overriding plate (bedrock) on which the land sits. When the accumulated strain at the boundary between the two plates reaches its limit, the fault shifts, causing a megathrust earthquake of around magnitude 8. Such earthquakes are known as "Kanto earthquakes." They have occurred repeatedly throughout history, an example of which is the magnitude-7.9 Great Kanto Earthquake.

These Kanto earthquakes were thought to have been the only major trench-type earthquake to strike the Kanto region repeatedly. However, research by a team from the National Institute of Advanced Industrial Science and Technology (AIST) suggests otherwise. 

Giant Earthquake in Boso, 1000 Years Ago

In a survey of tsunami sediments, the team discovered that a megathrust earthquake with an unprecedented magnitude of 8.5 occurred 1,000 years ago during the Heian Period (794-1185). This earthquake occurred off the eastern coast of the Boso Peninsula in Chiba. The team redefined the conventions of earthquake research when it published its findings in a 2021 paper.

With three overlapping plates, the Kanto region has a very complex subsurface structure. The uppermost plate is the overriding plate, beneath which the Philippine Sea Plate subducts from the southeast. Beneath that, the Pacific Plate subducts from the east in a three-layer structure.

Unlike Kanto earthquakes, the epicenter of the newly discovered megathrust earthquake was deep. It occurred where the Pacific Plate subducts beneath the Philippine Sea Plate. The scale of the quake was historically unprecedented. Investigations revealed that a massive tsunami swept inland as far as Kujukuri Beach on the Boso Peninsula.

'We Must Be Vigilant'

In 1677, during the Edo period, the Enpo Boso-Oki Earthquake (magnitude 8.0) caused widespread damage from the resulting tsunami. It is the largest earthquake to have occurred with an epicenter off the coast of Boso. The fact that another megathrust earthquake occurred in almost the same area of the ocean has important implications. 

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Masanobu Shishikura, leader of the Domestic Collaboration Group (Paleoearthquakes) in the Collaboration Promotion Office at AIST, has been researching earthquakes for many years. 

Speaking on the discovery, Shishikura said, "Whether the mechanism of occurrence is the same [as the Enpo Boso-Oki Earthquake] or not still needs to be verified. However, the fact that M8-class earthquakes have repeatedly occurred off Boso, where the Pacific Plate subducts, is a significant discovery." 

He also warned, "It has already been 350 years since the Enpo earthquake. Considering these earthquakes occur every few hundred years, we must be vigilant about when the next might strike."

Updating Prevention Measures

The government's Headquarters for Earthquake Research Promotion (HERP), releases long-term evaluations of major earthquakes in Japan, indicating the magnitude and probability of future occurrences. However, the singular nature of this newly discovered type of earthquake complicates any long-term evaluation of Boso earthquakes. Despite the pressing need for review, HERP has no plans to do so even now, two years after the publication of the AIST paper.

In the past, Shishikura and his colleagues discovered that the 869 Jogan earthquake triggered a massive tsunami in the Tohoku region and that another was imminent. However, the Great East Japan Earthquake, a similarly massive megathrust earthquake, occurred just before this knowledge could be incorporated into long-term evaluations.

Consequently, their findings were not used in the disaster prevention measures. We cannot afford to leave the Kanto region so vulnerable again.

"It is difficult to feel a sense of urgency over things that only happen once every several hundred years," Shishikura explains. "But the Great East Japan Earthquake happened nonetheless. We have to take this very seriously."

Tsunami Risk

Boso-Oki lies adjacent to the epicenter of the Great East Japan Earthquake. It is also identified as having a high risk of earthquakes due to the remaining fractures in the faults at the plate boundary.

Shishikura stresses, "The impact of the Great East Japan Earthquake [on the faults] means that a massive earthquake off the coast of Boso is even more imminent. There is a risk of a tsunami sweeping as far as two to three kilometers inland. We need to fully alert residents. This is the most important place to watch for trench earthquakes in the Kanto region."

Another Unknown Kanto Earthquake

But this is not the only unknown megathrust earthquake that could strike the Kanto region. There are fears of another M8 magnitude earthquake occurring along the Sagami Trough. Kanto earthquakes have two adjacent east-west epicenters. The 1703 Genroku Kanto earthquake (M8.2) occurred across the east and west sides. However, the Great Kanto Earthquake of 1923 occurred only on the west side near land. 

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The accumulated strain on the east side remains after the occurrence of earthquakes like the Great Kanto Earthquake. Experts know that a Genroku-type earthquake, which occurs only once every 1,000 to 2,000 years, would not fully alleviate this strain. This suggests a yet unknown earthquake may occur exclusively on the eastern side.

However, researchers have found no evidence that such an earthquake ever occurred. Firstly, offshore earthquakes are difficult to trace on the ground. Secondly, the Kanto region lacks historical records before the Edo period, making it difficult to find evidence of past earthquakes. Therefore, the mechanism of the unknown Kanto earthquake remains a crucial point of research in elucidating seismic activity in the Sagami Trough.

More Research Needed

Further peculiarities exist in the outer Boso region on the east side of the Boso Peninsula. Topographic uplift that cannot be explained by crustal deformation associated with the Kanto earthquake or the megathrust earthquake off Boso suggests the possibility of the existence of further unknown earthquakes.

"Considering the complex plate structure of the Kanto region," Shishikura explains, "there may be more earthquakes that could occur. We have a long way to go before we have a complete picture, but we hope to figure it out little by little."

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(Read the article in Japanese.)

Author: Yosuke Osanai