Ground-Motion Amplifications in Small-Size Hills: Case Study of Gokgang-ri, South Korea, during the 2017 M-L 5.4 Pohang Earthquake Sequence

Abstract

An earthquake with a local magnitude (M-L) of 5.4 occurred in Pohang city, South Korea, on 15 November 2017. This study focuses on the damage that affected the village of Gokgang-ri, which is built on small-size hills. The northern part of the village is located on the slopes facing the earthquake's epicenter, or on plateaus behind the slopes, and serious damage (i.e., building cracks and collapses) occurred to buildings in this area. In contrast, only some buildings in the southern part, which is located on the slopes that face the opposite direction, suffered minor damage, and no serious damage was incurred. We installed four seismic stations to record ground motions of several aftershocks (M-L 2.0-4.6); results showed amplification of ground motions in the northern part compared with the southern part. Given the similar geological and soil conditions in both parts, we conclude that more damage was incurred on the northern side because the ground motions were amplified in relation to constructive interference, which occurred when motions with large incidence angles encountered topographic surface irregularities. We demonstrate this using numerical simulations.