Saikaidō

is a Japanese term for both an ancient division of the country and the main road running through the region. 
History.
"Saikaidō" was one of the main circuits of the "Gokishichidō" system. It was first established during the Asuka period. Over centuries, the government functions of the "Gokishichido" became less important. However, the traditional eight regions, including "Saikaidō", remained cultural markers.
The "Saikaidō" road connected the capitals of the provinces in this region.
Geography.
"Saikaidō" is the geographic region of Kyūshū. Tsushima Island and Iki Province were also part of the region. 
"Saikaidō" has nine ancient provinces and two islands, including 
Regional earthquakes.
Historic earthquakes are named "Saikaidō" when the main damage was in the Kyūshū region. Often quakes are called by the Japanese era name along with region location such as "Saikaido".