Sentinel-1 change detection and precursor test, Langtang Lirung, 26 August 2026: derived data, figures and analysis code

Derived data, figures and analysis code from an independent Copernicus Sentinel-1 analysis of the rock-ice avalanche and flood that began on the west flank of Langtang Lirung, Nepal, on 26 August 2026.

The archive supports two results. First, post-event change detection locates the failure from radar alone: within 500 m of the published detachment coordinate, 30.0% of VV pixels and 25.4% of VH pixels changed by more than 6 dB, against 0.7% and 0.4% across the remainder of a 308 square kilometre scene. Second, a test for a precursory signal across five pre-event passes on a single relative orbit, measured against 80 terrain-matched control patches, finds no progressive pre-failure change. A monotonic rise in the cross-polarised channel was identified as a candidate and then rejected by a same-orbit seasonal baseline: the rise is the completion of the seasonal transition, and a ten-year climatology on the same relative orbit places 2026 within 0.3 dB of the 2017-2025 median across the analysis window.

Contents: the VV backscatter change raster in dB (GeoTIFF, EPSG:4326), five figures, a reproducible analysis script, and a README giving the exact export settings, area of interest, acquisition dates and product identifiers needed to regenerate the inputs from the free Copernicus archive.

The change raster shows where backscatter changed and is not a mapped failure scar; radar shadow, layover and snow cover bias it, and the threshold used is arbitrary. Radar amplitude is insensitive to slow deformation, so the negative precursor result does not demonstrate that no precursory movement occurred.

Note on the archived files: the README and the figure files in this record describe the source area as the north face and were generated before the aspect was measured from the source polygon (mean aspect 257 degrees). The corrected wording is used in the preprint. The files are otherwise correct and are retained unchanged so that the archived analysis remains exactly reproducible.

Contains modified Copernicus Sentinel data 2026, processed to orthorectified gamma-nought by Sentinel Hub. Elevation is SRTM, obtained as Mapzen / AWS Terrain Tiles.

How this was made: analysis code and manuscript drafting were carried out with assistance from Claude, an AI assistant made by Anthropic. All data acquisition, analytical decisions and interpretation are the author's, and the author is responsible for the content. The included script allows every reported result to be independently reproduced from the freely available source data.