This paper describes the relationship between bedrock depth (D) and site fundamental frequency (f₀) in the Nakdonggang delta region in the southeastern part of the Korean peninsula. We collected borehole logs to confirm the thickness of the sediments and estimated the f₀ at over 200 locations across the delta using the horizontal-to-vertical spectral ratio (HVSR) method. We developed an f₀ map of the study area by spatially interpolating the f₀ values using the Ordinary Kriging method. The bedrock depth in the main delta showed a power-law dependence on the f₀. The derived f₀–D model predicted much shallower bedrock depths compared with similar studies from other parts of the world. This was attributed to the fact that the Nakdonggang delta region is composed of relatively low V_s Holocene sediments. With an f₀ map, the derived model could enable a quick estimation of the bedrock depth, which could help to determine the site class in the Nakdonggang delta region according to the Korean Seismic Design Standard (KDS 17 10 00).

This study evaluates the effectiveness of the previously proposed Standard Penetration Test (SPT) N and shear wave velocity (V_S) models in relation to the geological attributes of the Busan region, situated in the southeastern part of the Korean peninsula. The multiple empirical N-V_S models, which used datasets collected from different regions in South Korea, resulted in distinct N-V_S trends across models. To validate the predictive capabilities of each model, this study gathered boring logs containing SPT N and V_S measurements within the Busan region, followed by a thorough residual analysis. The Busan area encompasses a delta region to the west and erosion basins within the mountains and hills to the east. Despite the relatively confined geographical scope, we found that models developed using data from fill areas exhibit superior performance for the delta region (western Busan), while models constructed from datasets within erosion basins perform better for the erosion basin region (eastern Busan). This comparative examination supports the dependence of the N-V_S model on geologic attributes and offers the valuable insight that N-V_S models developed with analogous geological attributes should be employed when estimating V_S from SPT N values.