The ″Median Tectonic Line″ causes a ″simultaneous! The “Median Tectonic Line” causes “Simultaneous!

More massive earthquakes loom

A major earthquake in the area that this magazine had feared has become a reality.

On April 20, a magnitude (M) 7.7 earthquake occurred off the coast of Sanriku, Hokkaido, along the Kurushima Trench and the Japan Trench. The town of Hashikami in Aomori Prefecture was hit by a tremor measuring 5-plus on the Japanese seismic scale, and Kuji City in Iwate Prefecture was hit by an 80-cm-high tsunami. The following commentary is by Hiroki Kamata, professor emeritus at Kyoto University, who warned in the April 17 and 24 issues of FRIDAY that earthquakes in the Kuril Islands Trench and the Japan Trench were imminent (comments below are his).

The Kuril Trench and the Japan Trench are located at the boundary between the Pacific Plate and the North American Plate, and have accumulated “strain” since a massive earthquake occurred about 400 years ago. In the future, both trenches may be subject to an M9-class super quake that surpasses the Great East Japan Earthquake, with strong tremors of intensity 7 and huge tsunamis of up to nearly 30 meters hitting the Pacific coasts of Hokkaido and Tohoku. It is known that seismic activity becomes more active inland before a huge earthquake occurs in the trench at the plate boundary. Caution should be exercised outside of coastal areas as well.”

Indeed, there have been many inland earthquakes in Hokkaido and Tohoku.’ The northern Miyagi Prefecture series of earthquakes in July 2003 (M6.4), the Iwate-Miyagi Nairiku earthquake in June ’08 (M7.2), and the eastern Hokkaido Iburi earthquake in September ’18 (M6.7). …… It is believed that the oceanic plate is subducting into the land plate, stimulating inland faults.

The crisis is not limited to eastern Japan.

Since the Great Hanshin-Awaji Earthquake (M7.3) in January 1995, inland seismic activity has been increasing in western Japan, including the Kumamoto Earthquake (M7.3) in April 2004 and the northern Osaka Prefecture Earthquake (M6.1) in June 2006 (see figure above). There is a super giant fault that requires particular attention. It is the Median Tectonic Line, one of the world’s largest aggregations of faults, stretching more than 1,000 km from Kyushu to Kanto.

Earthquakes that affect each other

Because the Median Tectonic Line is long, earthquakes frequently occur in many places. A huge earthquake could occur anywhere and at any time.

The characteristic feature is that it does not end with a single earthquake. For example, after the Keicho-Iyo earthquake (estimated M7.0) occurred on September 1, 1596, the Keicho-Bungo earthquake (M7.0) and the Keicho-Fushimi earthquake (M7.5) occurred in rapid succession within a few days. When the Median Tectonic Line or its extension shakes in one place, it affects other places, and huge earthquakes occur simultaneously.

The government’s Earthquake Research Commission has designated areas on the Median Tectonic Line as particularly dangerous. The fault line is about 440 km long, stretching from the Kinki region to Kyushu, and is located around Mount Kongo in Nara Prefecture, Gojo Valley in Wakayama Prefecture, Mount Ishizuchi in Ehime Prefecture, and Yufuin in Oita Prefecture.’ The Kumamoto earthquake in April 2004 also occurred on the Oita-Kumamoto Tectonic Line, which is continuous with the Median Tectonic Line. There is a risk that huge earthquakes may occur successively on these faults without a pause. This is the reason why the Median Tectonic Line is called the “nest of earthquakes.

Each municipality has estimated the number of fatalities in the event of a massive earthquake along the Median Tectonic Line. Nara Prefecture is estimated to have more than 4,300 fatalities, Ehime Prefecture up to 12,750, and Oita Prefecture up to 30,000. The “earthquake nest” could cause extensive damage throughout western Japan.

The Median Tectonic Line, which is threatened by a massive earthquake, and other inland earthquakes that have been occurring frequently seem to indicate that a massive M9-class Nankai Trough earthquake (hereafter referred to as the “Nankai Trough Earthquake”) is imminent. The Nankai Trough earthquake, which is said to cause not only earthquakes but also huge tsunamis that will sweep from Kyushu to Kanto and kill up to 290,000 people, is expected to occur around 2035, based on past ground uplift and other factors. In the past, a series of inland earthquakes occurred about 40 years before the Nankai Trough earthquake. In fact, there was a sharp increase in inland earthquakes before the 1944 Showa Tonankai Earthquake (M7.9) and the Showa Nankai Earthquake (M8.0) two years later (Figure, bottom).

The Nankai Trough earthquake is expected to affect 68 million people, or about half of the total population of Japan. The huge earthquakes along the Median Tectonic Line, which can be called “foreshocks,” are likely to occur simultaneously.

From the May 15 and 22, 2026 issue of FRIDAY