Topographic Ground Motion Amplifications in Pohang, South Korea

Abstract

In this study, we evaluated ground motion amplifications in the Pohang basin. We considered two topographic factors: (1) relative elevation (hr) divided into three sections (i.e., low, intermediate, high); and (2) relative angle between direction of slope and site-to-source direction (α). The α value of zero means that the slope direction is aligned with the direction towards the epicenter, and α of 180° means that the epicenter is located in the opposite direction to the slope aspect. We used 513 ground motions recorded at temporary stations deployed within the Pohang basin, South Korea. We derived the equations for 5%-damped spectral acceleration (SA) of the ground motions at 0.01–0.5 s period. We regressed the SAs against magnitude (M), epicentral distance (Repi), time-averaged shear wave velocity for surface to 30m depth (VS30), and the topographic factors. We observed that the SAs have strong correlations with M and Repi but do not have significant correlation with VS30. Furthermore, for the low section of hr, when the α is closer to zero, ground motions are de-amplified. For the high section of hr, when the α is closer to zero, ground motions are amplified. However, if the α is closer to 180°, ground motions are de-amplified. For the intermediate section of hr, ground motions do not change with α.