No detectable precursor in Sentinel-1 backscatter before the 26 August 2026 Langtang Lirung rock–ice avalanche, Nepal

Ajay Kumar Sah 

On 26 August 2026 a rock–ice avalanche on the north face of Langtang Lirung, Nepal, initiated a debris flow and flood that ran roughly 100 km down the Lhende Khola, Bhote Koshi and Trishuli. Several contemporaneous reports asserted that satellite radar had tracked a month of accelerating slope movement before failure; that assertion has not been substantiated in published analysis. Using Copernicus Sentinel-1 IW GRD amplitude on a single relative orbit, this note (i) locates the failure independently and (ii) tests directly for a precursory signal. Co- and cross-polarised change between 16 and 28 August is concentrated on the reported detachment coordinate: within 500 m, 30.0% of VV pixels and 25.4% of VH pixels changed by more than 6 dB, against 0.8% and 0.4% respectively across the remainder of a 308 km² scene. Across five pre-event passes from 29 June to 16 August, measured against 80 terrain-matched control patches, VV shows a nominally significant negative trend (−0.20 ± 0.03 dB cycle⁻¹ across independent control draws, p ≈ 0.02) that is produced entirely by the first scene of the series; excluding it, the remaining four dates are flat (+0.01 dB cycle⁻¹, p = 0.99). VH shows a monotonic rise (+0.62 ± 0.03 dB cycle⁻¹, p < 0.01, robust to exclusion of either endpoint) which is rejected by a same-orbit seasonal baseline: in 2025 the equivalent rise occurred earlier and had completed by late June, whereas in 2026 the face was 3.05 dB below the 2025 plateau at the same date and subsequently rose 2.87 dB. The apparent precursor is the completion of a delayed seasonal transition. No precursory signal is present in the freely available amplitude record at 12-day sampling; the detection limit is approximately 1 dB of progressive change over two months.