Addressing VS30 estimation challenges for ‘Shallow Rocky-Layered’ sites: A novel regression-based extrapolation approach for sites with rock layer within a depth of 30 m

Accurately estimation of V_S30 for shallow shear-wave velocity profiles (<30 m) is essential for seismic site classification in earthquake engineering. Traditional extrapolation models, such as the constant and linear extrapolation methods, do not specifically account for sites with shallow rock layers. To improve estimation accuracy, this study proposes a new extrapolation model specifically designed for such sites. Firstly, using borehole data from Xinjiang, China, it was demonstrated that traditional models often underestimate V_S30 for this type of site, with discrepancies exceeding 16 %. This highlights the importance of considering velocity gradient changes between soil and rock layers. The new model developed through regression analysis of data from 224 boreholes across China, improves V_S30 estimation accuracy and is validated through residual analysis. Additionally, the model's regional applicability is confirmed using data from KiK-net sites in Japan. The findings underscore the need for tailored extrapolation models that consider regional geological conditions, offering a more reliable method for estimating V_S30, critical for seismic site classification and ground motion prediction.