The also Sagami Trench, Sagami Megathrust, or Sagami Subduction Zone is a 210-mile long trough

Trough (geology)

In geology, a trough generally refers to a linear structural depression that extends laterally over a distance, while being less steep than a trench.A trough can be a narrow basin or a geologic rift....

 where the Kanto earthquake

Kanto earthquake

can refer to any of the historical earthquakes to occur in the Kantō region that originate from slippage in the Sagami Trough. Although the 1854 Ansei Great Earthquakes also struck in the region, this term is never used to refer to these quakes.It can refer to:...

s of 1703 (Genroku), 1855 (Ansei) and 1923 (Great Kanto or Taisho) arose. It stretches from the Boso Triple Junction

Boso Triple Junction

Boso Triple Junction is a triple junction off the coast of Japan; it is the only example of a Triple Trench Junction on earth.- Formation :...

 where it meets the Japan Trench

Japan Trench

__notoc__The Japan Trench is an oceanic trench, a part of the Pacific Ring of Fire, in the floor of the northern Pacific Ocean off northeast Japan. It extends from the Kuril Islands to the Bonin Islands and is at its deepest. It is an extension of the Kuril-Kamchatka Trench to the north and the...

 to Sagami Bay

Sagami Bay

Sagami Bay , also known as the Sagami Gulf or Sagami Sea, lies south of Kanagawa Prefecture in Honshū, central Japan, contained within the scope of the Miura Peninsula, in Kanagawa, to the east, the Izu Peninsula, in Shizuoka Prefecture, to the west, and the Shōnan coastline to the north, while the...

. It runs north of the Izu Islands

Izu Islands

The are a group of volcanic islands stretching south and east from the Izu Peninsula of Honshū, Japan. Administratively, they form two towns and six villages; all part of Tokyo. The largest is Izu Ōshima, usually called simply Ōshima....

 chain and the Izu-Bonin-Mariana Arc

Izu-Bonin-Mariana Arc

The Izu-Bonin-Mariana arc system is an outstanding example of a plate tectonic convergent boundary. IBM extends over 2800 km south from Tokyo, Japan, to beyond Guam, and includes the Izu Islands, Bonin Islands, and Mariana Islands; much more of the IBM arc system is submerged below sealevel...

 (IBM). The average quake the Sagami Trough releases is over 8.0 or higher. The Sagami Trough is a threat to Tokyo

Tokyo

, ; officially , is one of the 47 prefectures of Japan. Tokyo is the capital of Japan, the center of the Greater Tokyo Area, and the largest metropolitan area of Japan. It is the seat of the Japanese government and the Imperial Palace, and the home of the Japanese Imperial Family...

 and the Kanto Region

Kanto region

The is a geographical area of Honshu, the largest island of Japan. The region includes the Greater Tokyo Area and encompasses seven prefectures: Gunma, Tochigi, Ibaraki, Saitama, Tokyo, Chiba, and Kanagawa. Within its boundaries, slightly more than 40 percent of the land area is the Kantō Plain....

because of the proximity to a population center (with over 36 million living in Tokyo's metro area, with a total of 43 million living in the Kanto Region) and the magnitude the Sagami Trough can create.