Forecasting hanging glacier instabilities remain challenging as sensing technology focusing on the ice surface fails 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 15,000 m3 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 13.9(±1.2) m within 5 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. Plain Language Summary: Predicting the development of ice mass breaking off from unstable glaciers is challenging. Such glaciers are often located in remote places with steep terrain, making it difficult to gather observations. Because of that, our current view on ice damage development on unstable glaciers is incomplete. Here, we propose a new approach to tackle this problem using seismic observations from the Eiger hanging glacier in the Swiss Alps before a moderate 15,000 m3 break‐off event. We first group seismic signals according to their similarity. We then use waves scattered by fractures within the ice and at the glacier‐rock boundaries to track displacement of ice damage. Our results suggest that a group of seismic signals is generated by crevasse propagation separating the unstable ice mass and the stable ice uphill. Combined with a simple analytical model, these observations indicate that seismic source displacement can be associated with damage propagation within unstable glacier ice. This is an important step toward a better understanding of unstable ice flow and forecasting glacier instabilities. Key Points: In the months leading up to a break‐off event, we find thousands of recurring icequakes on the Eiger hanging glacier in SwitzerlandCoda wave interferometry resolves displacements of icequake multipletsOne multiplet displacement represents englacial damage corresponding to crevasse extension between unstable and stable ice masses [ABSTRACT FROM AUTHOR]