Since the beginning of November, earthquakes that directly lead to huge earthquakes have been occurring frequently,” said Mr. Kikuchi, a specialist in disaster forecasting at the University of Tokyo.
Manabu Takahashi, a specialist in disaster forecasting and a specially-appointed professor at Ritsumeikan University’s Research Center for Pacific Rim Civilizations, said, “Since the beginning of November, earthquakes directly related to major earthquakes have been occurring frequently. According to Takahashi, eerie tremors have been observed in areas that have not been hit by earthquakes very often.
November 20: M4.0 in southern Wakayama Prefecture
November 22: M2.8 in western Shizuoka Prefecture
The magnitude (M) of the quakes is small, but tremors in areas where there has been no movement so far should be noted. These have shallow epicenters and occur near the boundary between the Philippine Sea Plate and the Eurasian Plate. The next time it moves, the plate will bounce back and could lead to a huge Nankai Trough earthquake,” says Takahashi.
The Chubu and Kinki regions are not the only ones experiencing spooky tremors. A series of M4 earthquakes are also occurring in the northwestern part of Chiba Prefecture and the southern part of the Boso Peninsula. Takahashi continues.
The Chiba quake occurred near the boundary between the Philippine Sea Plate and the North American Plate, where the Sagami Trough is located. This is where the Sagami Trough is located. If a Sagami Trough earthquake occurs, there will be major damage in the Tokyo metropolitan area. I call the simultaneous movement of the Nankai Trough and Sagami Trough the ‘Super Nankai Earthquake,’ and the risk of its occurrence is increasing.
According to Takahashi, the following processes are expected to occur before the occurrence of a major earthquake.
1 A precursor earthquake (foreshock) occurs.
2 A quiet period of about 60 to 70 days.
3 A small earthquake at the same location as the foreshock.
4. A huge earthquake occurs about half a day to three days later.
In other words, we need to be careful from the end of January to February 2011. The Great Hanshin-Awaji Earthquake occurred in the same pattern, but there is a possibility that it will be a huge earthquake of M8.5 or greater.
Why Areas Far from the Epicenter Shake
There is one more case to watch out for. On November 14, a 6.1 magnitude earthquake occurred off the southeast coast of Mie Prefecture. Mie and Aichi prefectures near the epicenter hardly shook at all, but Fukushima Prefecture, about 500 km away from the epicenter, measured an intensity of 4 on the Japanese seismic scale. The reason for this is the depth of the epicenter, which was about 350 km (see figure). Geophysicist and Musashino Gakuin University professor Hidenori Shimamura explains.
An anomalous seismic zone is a phenomenon in which seismic waves propagate along plates and shake distant areas. Deep earthquakes with epicenters several hundred kilometers deep are more likely to occur. The Kinki and Chubu regions were not shaken much because of the geological strata directly above them, which prevents seismic waves from propagating. This time, however, the seismic waves propagated along the long Pacific Plate, and large tremors were observed in the Tohoku region.
What is frightening is the possibility that the anomalous quake zone could trigger an earthquake directly under the Tokyo metropolitan area.
The earthquake that will hit the Tokyo metropolitan area is related to the Pacific Plate. There is a theory that when stress is dissipated in the deep part of the plate, it stresses the shallow part. The Pacific Plate becomes shallower as one moves eastward. We cannot deny the possibility that this could be a trigger for an earthquake directly under the Tokyo metropolitan area,” Shimamura said.
The entire archipelago of Japan is threatened by a Super Nankai earthquake and an earthquake directly under the Tokyo metropolitan area. Makoto Okamura, professor emeritus at Kochi University and a member of the Cabinet Office’s “Nankai Trough Large Earthquake Model Study Group,” urges caution about tsunami.
In the Great East Japan Earthquake, there was at least a 25-minute delay before the tsunami reached land. In the case of the Nankai Trough earthquake, however, it takes three minutes for a tsunami to reach the land, as seen in Muroto City, Kochi Prefecture. This is due to the proximity of the wave source area where tsunamis are generated and the topography of the seafloor. The estimated death toll is more than 320,000. The total damage is estimated to be 220.3 trillion yen.
Mr. Okamura emphasizes the importance of prompt evacuation action above all else.
From the December 16, 2022 issue of FRIDAY
Photo by： Shun Kirishima