Kinematic analysis and fault-dependence of building-wall fracture patterns during moderate earthquakes

Two recent moderate earthquakes in South Korea, the 2016 M W 5.5 Gyeongju earthquake and 2017 M W 5.4 Pohang earthquake, caused damages to modern residential buildings. These events occurred with almost the same magnitude and duration in the same seismotectonic environment but exhibited remarkably different focal depths, faulting types, surface deformation, and especially structural damage features, but the reasons for these contrasts remain unknown. Furthermore, the building damage patterns are different from the natural damages, which have typical patterns depending on the fault types. It is important to understand the key reasons of these different phenomena to prevent destructive hazards from future earthquakes, particularly in densely populated intraplate regions. Here, we reveal the relationships between the geological-seismic parameters and earthquake damage features based on the patterns of building damage associated with these two events. During post-event urgent field surveys, we systematically observed en-echelon (or Riedel-type) sub-horizontal fractures in building walls associated with strike-slip motion and high-angle conjugate X-shaped fractures in building walls associated with predominantly reverse oblique-slip motion. We attribute the different patterns of earthquake damage to variations in faulting types and associated ground motions; strike-slip faulting resulting in horizontal shear and oblique-slip faulting yielding vertical ground motion. We argue that these interesting characteristics of building damage are mainly caused by stress conditions depending on the environmental change from the underground crust to the ground surface of free face. Our study highlights the importance of post-event investigations of earthquake damage to improve the level of seismic hazard assessment. Our findings from this study could serve as a reference for establishing proper anti-earthquake design and reinforcement for seismic protection.