Magnitude 9 or higher! The Day a Super Trench Earthquake Strikes Japan

Six years have passed since the Kumamoto earthquake occurred on April 14 and 16, 2004. More than 43,000 houses were totally or partially destroyed, up to 200,000 people were evacuated, and 276 people died as a result of the quake, which hit Kumamoto and Oita prefectures at a maximum depth of 7. The Kumamoto earthquake was an earthquake of the direct-under earthquake type caused by the movement of an active fault line, but in the past six years, direct-under earthquakes in Japan have become an even more alarming situation.

There is a possibility of an earthquake of a more severe magnitude than the Kumamoto quake occurring from Tottori to Hokuriku, Niigata, Kinki, Gifu, and Aichi prefectures,” warns Manabu Takahashi, a specially appointed professor at Ritsumeikan University’s Research Center for Civilizations of the Pacific Rim.

Right now, there are a series of M4 earthquakes in Kyoto and the Noto Peninsula. In 1891, the Nobi earthquake, the largest earthquake ever recorded, occurred in the Chubu region with a magnitude of 8.0. There is a possibility that an earthquake of this magnitude could occur.

In the Nobi earthquake, the fault ruptured from Fukui City through Gifu Prefecture to Aichi Prefecture, and the death toll reached 7,273. In the Mino region of Gifu Prefecture, the fault appeared on the surface of the earth, and its steps were as high as 6 meters. Trees collapsed, and the surrounding mountains were all bare. At this time, the collapse rate of houses around Gifu City, Ogaki City, and Ichinomiya City, Aichi Prefecture, exceeded 80%. An earthquake of this magnitude in an urban area would surely have been a catastrophe.

The mechanism by which the Nobi earthquake occurred is the same as the earthquakes now occurring in Kyoto and the Noto Peninsula. In the Japanese archipelago, the Tohoku region is pushed westward by the Pacific Plate, and the southwest region is pushed northward by the Philippine Sea Plate. This is why the Japanese archipelago is shaped like an inverted hiragana character “ku”. The specific areas where stress tends to accumulate are from Tottori to Hokuriku, Niigata, Kinki, Gifu, and Aichi. The fact that the stress-prone areas are under the influence of a large force is the reason for the frequent occurrence of earthquakes in Kyoto and the Noto Peninsula,” Takahashi observes.

Since the 2011 off the Pacific coast of Tohoku earthquake, the movement of the Pacific Plate has become more active, and the movement of the Philippine Plate has been triggered by the movement of the Pacific Plate. And the Philippine Sea Plate is also moving actively, following the movement of the Pacific Plate. The Philippine Sea Plate is a small plate on a global scale, and it is being pushed by the giant Pacific Plate behind it. The increased movement of these two plates has caused frequent earthquakes in Kyoto and the Noto Peninsula, and the stress may be relieved at once in the form of a huge earthquake such as the Nobi earthquake.

Especially in Nagoya, we should be very careful,” he said.

The fault lines that are known to have moved during the Nobi earthquake are limited to the Gifu and Ichinomiya cities. South of there, there is no clear evidence of movement. It probably did not move much. In Nagoya City, there is a fault that has not yet moved as an extension of the Nobi Earthquake. It is called the Horikawa Fault, which runs from the west side of Nagoya Castle to the west side of Atsuta Shrine. If this fault were to move significantly, the area west of Nagoya Castle, where the ground is weak, would be devastated.

As the Pacific Plate becomes more active, various strains will accumulate throughout the Japanese archipelago. The risk increases not only for earthquakes directly inland.

We must also be on the lookout for trench earthquakes such as the Nankai Trough. The trench-type Tonankai and Nankai earthquakes occurred in ’44 and ’46, but the huge inland earthquakes came before and after them.’ The Tottori earthquake (M7.2) in ’43, the Mikawa earthquake (M6.8) in ’45, and the Fukui earthquake (M7.1) in ’48 hit the Japanese archipelago.’

We should not underestimate the earthquakes that are occurring frequently now. This is because there is a possibility that they could lead to a giant subduction earthquake, and then to a super giant trench earthquake of magnitude 9 or greater, which would move from the Sagami Trough to the Nankai Trough and the Ryukyu Trench in a single series.