Non-destructive Diagnosis and Deterioration Mechanisms of a 6th Century Korean Rock-carved Standing Buddha Triad in Taean

The Rock-carved Standing Buddha Triad in Dongmun-ri, Taean, Republic of Korea, is a National Treasure of Korea carved directly into granitic bedrock, but its material deterioration and damage mechanisms remain insufficiently constrained. This study evaluates its lithological characteristics, deterioration condition and physical weakening through integrated material characterization and in situ non-destructive diagnosis. Detached granite fragments from the Buddha Triad and specimens from adjacent bedrock were examined using petrographic, mineralogical, geochemical and magnetic analyses. Deterioration mapping, ultrasonic velocity measurement, infrared thermography, portable X-ray fluorescence analysis and SEM-EDS verification were applied to assess surface damage, weathering grade, blistering and discoloration products. The Buddha Triad is composed of coarse-grained biotite granite that is petrographically and geochemically comparable to nearby bedrock, indicating a common geological origin. Physical deterioration occupied less than 10% of the surface, whereas red and black discoloration were more extensive, accounting for 9.38% and 7.98%, respectively. The average ultrasonic velocity of 1,734 m/s indicates a completely weathered condition, and the estimated uniaxial compressive strength corresponds to weak rock. The deterioration is attributed to long-term weathering, carving-related mechanical disturbance and moisture inflow into the protective shelter, providing a basis for drainage improvement, shelter management and long-term monitoring.