1,2,3,5,4- 1National Dong Hwa University, College of Environmental Studies, Department of Natural Resource and Environmental Studies, Hualien, Taiwan (jyyen@mail.ndhu.edu.tw)
- 2Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas, USA
- 3Department of Earth Sciences, University of Oregon, Eugene, Oregon, USA
- 4Department of Geosciences, National Taiwan University, Taipei, Taiwan
- 5Institute of Oceanography, National Taiwan University, Taipei, Taiwan
The July 2025 Matai'an (MTA) landslide, which created a large dam lake in the high mountains of eastern Taiwan, is one of the largest landslides in the 21st century and constitutes an impactful cascading land-surface hazard following regional earthquakes and intense precipitation from typhoons. The September 2025 dam breach caused fatalities and infrastructure damage, and the large volume of remaining landslide deposits poses long-term threats to downstream communities. The MTA event prompts investigation into whether such catastrophic events are coincidental or represent recurring phenomena in this rapidly uplifting, humid mountain range.
In this study, we integrate multi-temporal satellite imagery, historical and modern aerial photography, and high-resolution DEM topographic analysis to understand the MTA failure mechanism and regional landslide history. At the initiation zone, field and remote sensing observations reveal an extensive earthflow-type landslide complex, primarily composed of weathered and fluidized pelitic schist fragments with limited boulder-sized blocks. These materials originated near a marble-schist bedrock contact, where fracture zones act as groundwater conduits and promote weathering of pelitic schist into clay-rich, liquefiable material. Time-series analysis reveals strong seasonal variations and a decadal trend of increasing surface water retention (NDWI) and vegetation stress prior to failure, creating ideal conditions for producing weathered fine-grained materials that progressively reached saturation.
To accumulate approximately 300 million m³ of failable materials on over-steepened hillslopes in this rapidly uplifting terrain, we observe evidence for variations in channel-hillslope coupling that enable weathered materials to accumulate in abundance prior to the 2025 failure. Analysis of normalized channel steepness identifies a prominent knickpoint at the tributary junction where the dam lake formed. This knickpoint acts as a local base level, creating gentler upstream gradients that limit sediment connectivity and delivery. This configuration, combined with accelerated bedrock weathering, causes debris production to outpace river incision in the uplands of the catchment. Consequently, thick packages of weathered colluvium accumulate on hillslopes until mechanical thresholds are breached by earthquake ground shaking and typhoon triggers.
Our DEM-based inventory of historical landslides in MTA and nearby catchments reveals the signature and remnants of similarly sized ancient landslide complexes not yet evacuated by rivers. We identify several belts of comparable earthflow deposits preserved along equivalent lithological contacts in eastern Taiwan's Central Range, demonstrating that MTA-type events may be characteristic in this setting. Satellite and aerial imagery mapping since the 1940s provides evidence of repeated large landslide activity and decadal-scale rapid regeneration of slide-prone weathered materials. These findings reveal an extremely hazardous landscape where rapid bedrock weathering, coupled with transient river adjustments, generates large, periodic catastrophic landslides.
How to cite: Yen, J.-Y., Lai, L. S.-H., Roering, J., Lin, L.-H., Wang, P.-L., and Lien, W.-Y.: Geologic and geomorphic controls on the 2025 Matai’an landslide and downstream impacts, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-8931, https://doi.org/10.5194/egusphere-egu26-8931, 2026.
