VS30 Map for Mainland China Constrained by Local VS30 Measurements and Incorporating Topographic Slope as Secondary Data via a Cokriging (SCK) Model

V_S30 is a widely used parameter for characterizing local site conditions. A V_S30 map serves as a fundamental dataset for various studies related to seismic hazard assessment and seismic risk mitigation. In China, over 30 years of extensive engineering projects have generated a wealth of borehole-based V_S measurements. We complied a site profile dataset containing tens of thousands of borehole profiles derived from over 3,000 site investigation reports associated with engineering projects across mainland China, covering all provinces and over 200 major cities. To better utilize this abundant site data for developing a V_S30 map for China, we proposed a Cokriging-based V_S30 proxy model (SCK model) in 2022 that uses V_S30 measurements as constraints and topographic slope as a secondary variable, producing the first version of the V_S30 map for mainland China. In 2024, we refined the SCK model by: (1) explicitly accounting for the influence of surrounding topographic slopes rather than only the slope at a single point, (2) incorporating more distant V_S30 measurements, and (3) improving the spatial distribution of V_S30 measurements used in the model calculation. Additionally, we added new V_S30 data from China strong-motion stations to enhance data coverage in western China. Using the refined SCK model and the expanded V_S30 data set, we developed the 2024 version of V_S30 map for mainland China. This map is constrained by 7,939 V_S30 measurements, features a grid resolution of 30 arcsec (approximately 900 m), and demonstrates reduced estimation errors with improved spatial continuity. Compared to the China part of the USGS Global V_S30 Mosaic, our map provides more plausible results due to its use of local data constraints and its better reflection of the geological and geomorphological characteristics of China. The 2024 version of the V_S30 map for mainland China is available as an open-access dataset to support further research and practical applications. We are currently working on incorporating surface geology and depositional environment as additional parameters to further enhance the SCK model’s performance and the map’s ability to reflect local site conditions.