Earthquake damage and related factors associated with the 2016 ML = 5.8 Gyeongju earthquake, southeast Korea

The Gyeongju area covers the intersection of two major young structural features in Korea: the Yangsan and Ulsanfaults. More than 60 Quaternary fault sites have recently been reported along these fault zones, which are thus considered major active tectonic features in southeast Korea. An earthquake of local magnitude ML = 5.8 struck the Gyeongju area on September 12, 2016; the largest instrumental earthquake recorded in South Korea. We performed detailed investigation for severely damaged buildings and houses in villages around the epicenter, and determined the characteristics and controlling factors of the earthquake damage. The distribution of damaged buildings is relatively scattered around the epicenter, which may be related to the relatively deep focal depth of approximately 13–15 km. The radius of the reported damage area affected by ground motion is approximately 17 km from the epicenter, which is almost equal to the focal depth. Old buildings with traditional styles are more seriously damaged than modern buildings, suggesting that the damage intensity depends on the building structure, material properties, and seismic design. Interestingly, in a small village, the degree of building damage is clearly divided by a small stream. Based on an electrical resistivity survey for the local geological condition, we found that the degree of building damages also strongly depends on the local unconsolidated alluvium thickness. Moreover, the orientation of tilted or damaged buildings is closely related to the general trend of the related faults indicating the propagation direction of ground motion. Although focal depth, building style, and foundation condition are the main controlling factors to the building damages caused during the 2016 Gyeongju earthquake, other minor factors could also be involved. This information will be useful to design proper construction codes for reinforced buildings and for hazard studies against future earthquakes in potential earthquake prone areas.