Cascading Hazards and Dynamic Evolution of the 2025 Mataian Giant Landslide Dam: From Earthquake-Induced Initiation to Catastrophic Breach and Residual Dam Instability

After the 2024 M_w 7.4 Hualien earthquake in eastern Taiwan, the Mataian River watershed experienced a catastrophic sequence of cascading geohazards. This study reconstructs the long-term evolution and failure kinematics of the 2025 Mataian giant landslide and its subsequent dam-breach events. By integrating multi-temporal LiDAR-derived topography, satellite imagery, microseismic signal analysis, and high-resolution UAV surveys, we offer a comprehensive geomorphic and kinematic reconstruction of this complex event.   Satellite images are identified a 1,200 m-long tension crack developing along the crown of a paleo-landslide after the 2024 earthquake. On 21 July 2025, a massive failure occurred with a maximum scarp retreat of 120 m and a failure depth of 380 m. Multi-temporal LiDAR differencing estimates a total landslide volume of ~308 million cubic meters. Microseismic records captured a distinct two-stage runout process: an initial dominant southeastward motion toward the Wang Creek tributary, followed by a secondary southward runout ~80 s later along the Mataian River mainstream. The resulting landslide dam reached a height of ~200 m and a maximum depositional thickness of ~325 m.    On 23 September 2025, the dam catastrophically breached, with the impounded lake volume plummeting from 91 to 1.15 million cubic meters and causing 19 fatalities and 5 missing persons downstream. Post-breach UAV observations of the residual dam exposed a stratified internal structure of fractured greenschist, quartz-mica schist, and marble, overlain by boulder-gravel deposits layer. Notably, subsequent failures on 21 October and 13 November were concentrated on the right bank. Due to the run-up process during the major event, where the colluvial front collided with the opposing slope, forming a steep and mechanically weak interface.   A comprehensive dynamic model of the landslide-to-breach sequence is established. Our findings provide critical insights into the post-failure stability of residual dams and important information for subsequent numerical modeling, physical breach experiments, and the hazard mitigation strategies in similar region.