Abstract: On August 10, 2026, a Mw 7.4 intermediate-depth strong earthquake struck western Colombia, with a focal depth ranging from approximately 96 km to 107 km. It is an intraslab rupture-type earthquake induced by the subduction of the Nazca Plate beneath the South American Plate. This earthquake exhibited an inverse disaster-distribution pattern: moderate damage in the mountainous epicentral area versus severe destruction in far-field river-valley urban agglomerations. The event caused 241 fatalities and 3 771 injuries. Numerous old masonry buildings without seismic detailing experienced progressive collapse, while lifeline engineering facilities and historical-cultural heritage sites sustained varying degrees of damage. Combined with source-rupture characteristics and ground-motion simulation results, this paper analyzes the distinctive disaster-formation mechanism of this intermediate-depth earthquake from four perspectives: large-scale uniform ground-motion output from the seismic source, amplified horizontal effects caused by near-vertical incidence of seismic waves, 30-s-level long-duration strong ground motion induced by the coupling of rupture processes and site conditions, and cumulative damage of brittle masonry structures under cyclic loading. The study reveals that the disaster risk of intermediate-depth earthquakes hinges not on instantaneous peak ground acceleration, but on structural damage accumulation induced by long-duration cyclic loads. The Colombian government activated the highest-level national emergency response, implemented search-and-rescue operations and resettlement, repaired lifeline systems, prevented secondary hazards, and accepted international humanitarian assistance, forming a domestically-led and internationally-coordinated emergency-response mode. This case provides references for seismic fortification, retrofitting of old buildings, and disaster-risk management for river-valley towns in global subduction zones facing intermediate-depth earthquakes.