  We present a long-range fiber-optic environmental deformation sensor based on
active phase noise cancellation (PNC) in metrological frequency dissemination.
PNC sensing exploits recordings of a compensation frequency that is commonly
discarded. Without the need for dedicated measurement devices, it operates
synchronously with metrological services, suggesting that existing
phase-stabilized metrological networks can be co-used effortlessly as
environmental sensors. The compatibility of PNC sensing with inline
amplification enables the interrogation of cables with lengths beyond 1000 km,
making it a potential contributor to earthquake detection and early warning in
the oceans. Using spectral-element wavefield simulations that accurately
account for complex cable geometry, we compare observed and computed recordings
of the compensation frequency for a magnitude 3.9 earthquake in south-eastern
France and a 123 km fiber link between Bern and Basel, Switzerland. The match
in both phase and amplitude indicates that PNC sensing can be used
quantitatively, for example, in earthquake detection and characterization.
