A linguistic fault line
The Tsugaru Strait, a 19.5-kilometer-wide channel separating Japan's main island of Honshu from Hokkaido, is a body of water. It is a sharp linguistic boundary. On the Honshu side, in Aomori Prefecture, people speak the Tsugaru dialect (Tsugaru-ben), a branch of the Tōhoku dialects known for being difficult for outsiders to understand. Cross the strait to Hokkaido, and the local dialect becomes markedly closer to standard Japanese.
The differences are substantial. Tsugaru-ben features unique vocabulary, such as "megoi" for "cute" (kawaii in standard Japanese) and "keyagu" for "friend" (tomodachi). Pronunciation also diverges; speakers may not distinguish between sounds like "shi" and "su," or "chi" and "tsu." The dialect is often spoken quickly and can be so divergent that even people from the eastern side of Aomori Prefecture have trouble understanding it. In contrast, the Hokkaido dialect, while having its own local words—some borrowed from the indigenous Ainu language—is largely mutually intelligible with the standard Japanese spoken in Tokyo. This abrupt shift, with little gradual blending over the short distance, points to a historical, not geographical, cause.
An echo of 1869
The linguistic divide is a direct result of modern history. Before 1869, Hokkaido was known as Ezo and was primarily inhabited by the indigenous Ainu people, with only a small population of ethnic Japanese (Wajin) confined to the island's southern tip. The dialects on either side of the strait were more closely related. However, in 1869, the new Meiji government launched a massive colonization effort, renaming the island Hokkaido and establishing the Development Commission (Kaitakushi).
This program encouraged large-scale immigration to Hokkaido from all over Japan, not just from neighboring Aomori. Settlers from diverse regions brought their own dialects, and in the newly established capital of Sapporo and other inland settlements, a more standardized form of Japanese became the common language for communication, administration, and education. This process created a new, relatively uniform Hokkaido dialect based on standard Japanese, effectively overriding the influence of the geographically close but linguistically distinct Tsugaru dialect. The strait became a linguistic barrier, preserving the older, more isolated dialect on Honshu while a new linguistic melting pot formed on the northern island. Coincidentally, the strait also contains a famous zoogeographical boundary called the Blakiston Line, which separates many of Honshu's animal species from the distinct, northern-Asian-related fauna of Hokkaido.