At that time, there was a small continent to the north of present-day China. There was a trench in the southern part of this continent, and rocks on the sea floor were sinking under the continent.
When the undersea rocks settle under a continent, they are lost because they go under the continent. However, the sediments attached to the rocks stick to the continent, and the continent gradually grows larger as the sediments are carried in.
And this continued for about 300 million years. During this 300 million years, the first continent grew so large that it became the foundation of the Japanese archipelago.
Then, about 15 million years ago, the rocks that had melted underground began to rise up at the edge of the continent. This caused severe volcanic activity, and the continent broke off in the middle of what is now the Sea of Japan. The crevice spread out and finally became an ocean. This is the Sea of Japan.
In other words, Japan was originally part of the continent, but was cut off from it.