Unraveling the cause of slump during the 2017 Pohang earthquake: insights from geological and numerical analyses

A slump occurred on a slope, consisting of an artificial fill on top of a natural gentle slope, near the epicenter of the 2017 Pohang earthquake with a local magnitude of 5.4. The cause of the slump remains unresolved due to the uncertain surface projection of a blind seismic rupture and the concentration of surface deformation. These factors complicate the interpretation of slope response to seismic events, as they may influence the direction and magnitude of ground motion amplification, thus affecting slope stability. This study aims to examine the relation of the observed slump to the earthquake-induced ground shaking. A field observation with an unmanned aerial vehicle-derived LiDAR image and a trench survey is employed to identify the slump deformation and comprehend the subsurface geological structure of the area. Additionally, we conduct numerical simulations with a few models accounting for the effects of the fill and ground shaking on the slump deformation to understand the cause of the slump occurrence. Subsidence (extensional crack) and uplift (contractional bulge) are observed in the crest and toe of the slope, respectively, suggesting rotational sliding. Contours of displacements in horizontal and vertical directions and deviatoric strain from the numerical model considering the ground shaking effect align with the observed slump deformation, implying that the slump could be induced by the earthquake. This finding highlights the importance of considering blind fault geometry and localized ground deformation in seismic slope hazard assessment and geotechnical design.