Calibrated detection limits in routine Sentinel-1 products 50.6 hours before the Langtang Lirung rock–ice avalanche, Nepal

Ajay Kumar Sah 

On 26 August 2026 a rock–ice avalanche detached from Langtang Lirung, Nepal, and released a debris flood that crossed the Nepal–China border. Deformation preceding the failure has since been reported from a multi-month Sentinel-1 stack, and pre-event indications have been identified in commercial optical imagery. Both analyses began from a known failure location. This paper asks a different question: what routine, freely available Sentinel-1 products contained at the last acquisition before failure, and how large a signal would have had to be before they contained something. Three quantities are tested on a 500 m disc over the detachment — backscatter amplitude, interferometric coherence, and line-of-sight displacement from unwrapped phase — each against 120 control discs in the same scene matched on local incidence angle, aspect and slope, and against the source’s own history over the preceding two months. The last usable descending acquisition precedes failure by 50.6 hours. No quantity distinguishes the source from comparable slopes or from its own quiet behaviour: amplitude at the 67.5th percentile of controls, coherence third of five quiet intervals, cumulative displacement at the 31st percentile. The same statistics identify the collapse decisively once it falls inside the observation interval, at z = 4.57 in coherence and below all 120 controls in amplitude, so these are calibrated limits rather than uninformative nulls. No surface disturbance above approximately 2 ha of coherent decorrelation occurred in the twelve days ending 50.6 hours before failure, and no line-of-sight displacement above approximately 13 mm per month occurred over the preceding seven weeks. That bound lies above the rate reported from the multi-month stack, which is the substantive result: the deformation was measurable from orbit using free archived data, but not by the twelve-day products a monitoring service would have been screening.