Since Great East Japan Earthquake, I have been worried about the arrival of major earthquake, mainly in the Nankai Trough. I would like to consider the stratum of Chuo-ku based on the estimated seismic intensity in Tokyo during the Great Kanto Earthquake.
Edo, including Yamanote and Shitamachi, has a history of reclamation and moat creation. Immediately after the opening of Edo, the Nihonbashi and Ginza districts were created by the landfill of Edo Maejima, which continued from the Hongo Plateau, and the Hibiya Inlet was reclaimed to create the Samurai residence area of Hibiya, Yurakucho and Marunouchi districts. In the Edo period, the Tsukishima and Harumi districts were seas.
Let's take a look at the seismic intensity distribution of Tokyo Prefecture during the Great Kanto Earthquake. I'm worried about the red numbers (equivalent to seismic intensity 7) of Akasaka Tameike, Azabujuban and Koto-ku.
Tsukuda Island, which was not reclaimed and created, is a land created by naturally depositing sediment flowing from the upstream of the Sumida River, so the altitude is higher than the Tsukishima and Kachidoki districts. It seems that the seismic intensity at the time of the Great Kanto Earthquake was about 5. On the other hand, the Tsukishima, Kachidoki, and Harumi districts, which are land reclaimed using soil dredged on the Sumida River, are said to have had a seismic intensity of about 5-6 during the Great Kanto Earthquake.
Because it was a landfill, I had a preconception that the seismic intensity at the time of the Great Kanto Earthquake in the Tsukishima and Kachidoki districts might be even larger, but I am surprised at the size of Ginza and Nihonbashi.
There was a reef called Edo Maejima in Chuo-ku today. On the west side, there was Hibiya inlet from Hibiya to Otemachi. Immediately after the opening of the prefecture, the Shogunate reclaimed the land.
It is said that the ground on the east side of the Sumida River is bad and the ground on the west side is good. However, there is a hole in the flood area, and alluvial ground enters there, so there is a severe seismic area on the Otemachi side. On the other hand, the good ground of Edo Maejima is clear.
If you buy real estate, look at the old map and check the terrain. Don't be danced only by the place name "Safe in the mountains, downtown is dangerous"! The date of the Great Kanto Earthquake = September 1 was a word.
Reference: Masayuki Takemura (2003): Central Tokyo (formerly 15 wards) due to the 923 Kanto Earthquake Detailed seismic intensity distribution and surface ground structure, "Thesis of the Earthquake Engineering Society of Japan", Vol. 3, No. 1
