Direct tsunami wavefield sensing on submarine telecom cables enables coastal wave height forecasting

Detecting tsunamis in the open ocean before they reach the coast is critical for saving lives, yet open-ocean observations remain sparse and costly. Submarine telecommunication cables span many tsunami-prone regions and can be instrumented with distributed sensing, but their tsunami sensitivity remains uncertain. Here we report observations of the transoceanic tsunami generated by the Mw 8.8 Kamchatka earthquake on 29 July 2025, obtained through distributed acoustic sensing (DAS) on a 400-km telecommunication cable array offshore Chile. After suppressing background and instrumental noise, we recover coherent tsunami signals along hundreds of kilometres of cable despite maximum open-ocean amplitudes of only ~ 4 cm. The recovered signals agree with tsunami simulations, while revealing spatial variability in cable sensitivity. Converting DAS strain to water elevation empirically, we reconstruct the tsunami wavefield and predict coastal tide-gauge records where constrained by high-fidelity cable segments, providing 5 to 15 minutes of warning time before the tsunami arrives at the coast. These results demonstrate the viability of an end-to-end tsunami warning system based on existing submarine cables.