Forecasting of hanging glacier instabilities remains challenging as sensing technology focusing on the ice surface fail to detect englacial damage leading to large-scale failure. Here we combine icequake cluster analysis with coda wave interferometry constraining damage growth on Switzerland’s Eiger hanging glacier before a 15,000m3 break-off event. The method focuses on icequake migration within clusters rather than previously proposed “event counting”. Results show that one cluster originated from the glacier front and migrated by 13(+/- 4) m within five weeks before the break-off event. The corresponding crevasse extension separates unstable and stable ice masses. We use the measured source displacement for damage parametrization and find a 90% agreement between an analytical model based on damage mechanics and frontal flow velocities measured with an interferometric radar. Our analysis provides observational constraints for damage growth, which to date is primarily a theoretical concept for modeling englacial fractures.