Nankaidō in the context of "Awa province (Tokushima)"

Awa Province (阿波国, Awa no Kuni; Japanese pronunciation: [aꜜ.wa (no kɯ.ɲi)]) was a province of Japan in the area that is today Tokushima Prefecture on the island of Shikoku. Awa was bordered by Tosa, Sanuki, and Iyo Provinces. Its abbreviated form name was Ashū (阿州). In terms of the Gokishichidō system, Awa was one of the provinces of the Nankaidō circuit. Under the Engishiki classification system, Awa was ranked as one of the 35 "superior countries" (上国) in terms of importance, and one of the "middle countries" (中国) in terms of distance from the capital. The provincial capital was located in what is now the city of Tokushima.

Tosa Province (土佐国, Tosa no Kuni; Japanese pronunciation: [toꜜ.sa (no kɯ.ɲi), to.saꜜ (no kɯ.ɲi)]) was a province of Japan in the area of southern Shikoku. Tosa bordered on Awa to the northeast, and Iyo to the northwest. Its abbreviated form name was Doshū (土州). In terms of the Gokishichidō system, Tosa was one of the provinces of the Nankaidō circuit. Under the Engishiki classification system, Tosa was ranked as one of the "middle countries" (中国) in terms of importance, and one of the "far countries" (遠国) in terms of distance from the capital. The provincial capital was located in what is now the city of Nankoku. The ichinomiya of the province is the Tosa shrine located in the city of Kōchi.

Sanuki Province (讃岐国, Sanuki no Kuni; Japanese pronunciation: [saꜜ.nɯ.kʲi (no kɯ.ɲi), sa.nɯ.kʲiꜜ (no kɯ.ɲi)]) was a province of Japan in the area of northeastern Shikoku. Sanuki bordered on Awa to the south, and Iyo to the west. Its abbreviated form name was Sanshū (讃州). In terms of the Gokishichidō system, Sanuki was one of the provinces of the Nankaidō circuit. Under the Engishiki classification system, Sanuki was ranked as one of the "upper countries" (上国) in terms of importance, and one of the "middle countries" (中国) in terms of distance from the capital. The provincial capital was located in what is now the city of Sakaide, but its exact location was only identified in 2012. The ichinomiya of the province is the Tamura jinja located in the city of Takamatsu.

Iyo Province (伊予国, Iyo no Kuni; Japanese pronunciation: [iꜜ.jo (no kɯ.ɲi)]) was a province of Japan in the area of northwestern Shikoku. Iyo bordered on Sanuki Province to the northeast, Awa to the east, and Tosa to the south. Its abbreviated form name was Yoshū (予州). In terms of the Gokishichidō system, Iyo was one of the provinces of the Nankaidō circuit. Under the Engishiki classification system, Iyo was ranked as one of the "upper countries" (上国) in terms of importance, and one of the "far countries" (遠国) in terms of distance from the capital. The provincial capital was located in what is now the city of Imabari, but its exact location is still unknown. The ichinomiya of the province is the Ōyamazumi Shrine located on the island of Ōmishima in what is now part of Imabari. The people spoke Iyo dialect.

The Nankai Trough (南海トラフ, Nankai Torafu; Southern Sea Trough) is a submarine trough located south of the Nankaidō region of Japan's island of Honshu, extending approximately 900 km (559 mi) offshore. The underlying fault, the Nankai megathrust, is the source of the devastating Nankai megathrust earthquakes, while the trough itself is potentially a major source of hydrocarbon fuel, in the form of methane clathrate.

In plate tectonics, the Nankai Trough marks a subduction zone that is caused by subduction of the Philippine Sea plate beneath Japan, part of the Eurasian plate (Kanda et al., 2004). This plate boundary would be an oceanic trench except for a high flux of sediments that fills the trench. Within the Nankai Trough there is a large amount of deformed trench sediments (Ike, 2004), making one of Earth's best examples of accretionary prism. Furthermore, seismic reflection studies have revealed the presence of basement highs that are interpreted as seamounts that are covered in sediments (Ike, 2004). The northern part of the trough is known as the Suruga Trough, while to the east is the Sagami Trough. The Nankai Trough runs roughly parallel to the Japan Median Tectonic Line.