Both of these statements mean the same thing. Here "Zion" and "Jerusalem" both represent the people who live in Jerusalem. Alternate translation: "For the sake of the people of Jerusalem I will not be silent" (See: [[rc://en/ta/man/jit/figs-parallelism]] and [[rc://en/ta/man/jit/figs-metonymy]])
Both clauses reassure the people that God will eventually come and save the people of Israel and that it will be as apparent as light is. (See: [[rc://en/ta/man/jit/figs-simile]] and [[rc://en/ta/man/jit/figs-parallelism]])