Estimation of Shear Wave Velocity (Vs30) Using the Monte Carlo HVSR Inversion Method on the North Coast of Bengkulu, Indonesia

Assessing seismic site conditions through the time-averaged shear-wave velocity of the upper 30 meters (Vs₃₀) is fundamental for understanding local geological response and supporting seismic microzonation. This study investigates the subsurface characteristics of the Ketahun region, North Bengkulu, by integrating the Horizontal-to-Vertical Spectral Ratio (HVSR) method with Monte Carlo inversion to obtain a robust estimation of Vs₃₀. A total of 29 microtremor measurement stations were deployed using a three-component seismometer, and ambient vibration data were processed with Geopsy to derive dominant frequency (f₀) and site amplification (A₀). These parameters served as key constraints in the inversion procedure to construct shear-wave velocity profiles. The inversion results yield Vs₃₀ values ranging from 257 to 1220 m/s, indicating substantial lateral heterogeneity driven by variations in sediment thickness and bedrock depth. Lower Vs₃₀ values characterize coastal alluvial deposits, whereas higher values are found in the eastern–northern hilly terrain dominated by shallow bedrock. A comparative assessment with the global USGS topographic-slope-based Vs₃₀ model reveals systematic discrepancies, with the USGS proxy consistently underestimating Vs₃₀ in hard-rock domains. Statistical evaluation confirms that the differences between both datasets are significant, underscoring the superior representativeness of HVSR-based inversion for local-scale site characterization, particularly in geologically complex environments. Overall, the findings demonstrate the effectiveness of the HVSR Monte Carlo approach in resolving near-surface velocity structures and highlight its importance for seismic hazard assessment, microzonation efforts, and regional planning in North Bengkulu. The resulting Vs₃₀ distribution provides a valuable foundation for future geotechnical validation and for the integration of complementary subsurface investigation methods.