Secession would be the word your looking for
Most of the Japanese people on flatland called the plains.
Explanation:
Although japan is a vast country by landmass it has an incredibly dense [population as very littler of the landmass of the country is actually inhabitable for the people and most people live in the smell area in the middle in the bigger islands of the country which is the plain area that is a flatland.
Most Japanese people live in the flatland because it is easier to live there away from all the natural difficulties that dominate life in other places.