The voltage and frequency are independent of each other.

A quick web search revealed this in the first result: https://www.electricaltechnology.org/2025/02/japan-50hz-60hz.html

This division originates from the 1880s, when Tokyo imported 50 Hz generators from Germany, while Osaka chose 60 Hz equipment from the United States.

Eastern Japan operates at 50 Hz, while Western Japan operates at 60 Hz. This division originates from the 1880s, when Tokyo imported 50 Hz generators from Germany, while Osaka chose 60 Hz equipment from the United States.

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Standardizing the frequency nationwide would have required replacing a vast amount of existing infrastructure, which was deemed impractical and costly.

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Fun fact: the two grids share power over a back to back HVDC system, so they're technically one big weird grid.