First Application of Fragile Geologic Features for Paleoseismic Constraints in South Korea

Precariously balanced rocks (PBRs), a subset of fragile geologic features, preserve constraints on paleoseismic activity through their susceptibility to sliding and toppling under strong ground motion. This study analyzed six PBRs on Mt. Su-am and Mt. Yongbong, located near major metropolitan areas in central South Korea. Using three-dimensional geometries constructed through structure from motion, we estimated the minimum peak ground acceleration thresholds required to destabilize each PBR (i.e., fragility). Fragility ages, representing the maximum timing of paleoearthquakes, were determined based on in situ cosmogenic radionuclide (10Be) concentrations measured along vertical sections. Sliding fragility ranged between 2 percent and 91 percent g, and the toppling fragility ranged between 25 percent and 273 percent g, with best-fit fragility ages spanning 0.85 to 69.73 ka. Additionally, modeling of 10Be depth profiles reveals that high pre-exhumation concentrations support two-stage or intermediate models of tor formation. This result also highlights the influence of local geomorphic settings, such as elevation and location of PBRs, on exposure histories. Our findings enhance the understanding of tor formation, which contributes to the use of PBRs in seismic hazard assessments. Moreover, sliding fragility, previously unexplored, effectively detects moderate-magnitude paleoearthquakes. These results expand the utility of PBRs in studying lower magnitude paleoseismic events and into urbanized areas where other paleoseismic methods might not be applicable due to widespread ground disturbance.