Characterizing the Modes of Earth’s Free Oscillations via Submarine Distributed Acoustic Sensing (DAS)

Distributed acoustic sensing (DAS) on seafloor telecommunications cables is emerging as a powerful approach for observing geophysical processes. However, instrumental noise increases steeply below 10 millihertz (mHz), limiting the resolution of long-period signals. Here, we overcome this limitation by isolating low-frequency common-mode noise using channels with minimal signal amplitude, enabling the observation of low-frequency spectral peaks consistent with Earth’s normal-mode oscillations in the mHz band along a submarine fiber-optic cable in the western Mediterranean Sea. Following the 2025 Mw 8.8 Kamchatka earthquake, we observe systematic variations in modal amplitudes with cable azimuth over 120 km, consistent with predictions from a normal-mode summation model. The amplitude is predicted above the noise level, but the signal is only observed after noise removal. This is explained by an event-independent response coefficient (C ∼ 0.13), consistent with a flat instrumental response. Normal modes are also detected for smaller earthquakes down to Mw 7.7. The variably oriented cable resolves the radial and transverse horizontal components of motion. The large-scale signal’s amplitude is locally modulated by variable cable azimuth, demonstrating a sensitivity for cable relocation. Our results demonstrate that DAS can resolve subtle solid-Earth deformation at mHz frequencies, opening new opportunities for large-scale, ocean-based monitoring of Earth structure and dynamics.