By tracking the state of polarization in submarine optical cables carrying live traffic across the Mediterranean Sea, we unambiguously detected the signature of the recent $M_w$ 8.8 Kamchatka earthquake in two independent optical links. Moreover, distinct precursors of the earthquake were observed in multiple cables within the same region. Their distance of roughly 9,000~km from the epicenter unequivocally indicates that the earthquake preparation was governed by Earth-scale processes, whose nature remains unknown. We conjecture that the excitation of Earth’s normal modes by solid tides may explain this global-scale behavior. These results demonstrate the potential of submarine optical fiber links as highly sensitive geophysical observatories, opening new perspectives for understanding earthquake dynamics and developing future early warning systems.