SOIL VULNERABILITY ANALYSIS FOR SEISMIC ZONATION IN THE NUSA TENGGARA TIMUR REGION USING THE HORIZONTAL TO VERTICAL SPECTRAL RATIO (HVSR) METHOD

Abstract

Nusa Tenggara Timur (NTT), located within the seismically active Sunda–Banda Arc transition zone, is characterized by complex tectonic interactions involving the Indo-Australian, Eurasian, and Pacific plates. To support earthquake hazard miti- gation in this region, this study applies the Horizontal to Vertical Spectral Ratio (HVSR) method to analyze soil vulnerability using ambient seismic noise recorded at 28 YS network stations. Seismic waveform data were filtered (0.1–10 Hz), trans- formed using Fast Fourier Transform (FFT), and Power Spectral Density (PSD) method to construct Horizontal to Vertical Spectral Ratio (HVSR) curves. From these, dominant frequency (f₀), amplification factor (A₀), and soil vulnerability in- dex (Kg) were derived and correlated with geological formations, Vs₃₀ values, and seismicity patterns. The results show high f₀ values (>8 Hz) with low A₀ and Kg in areas of hard rock (e.g., volcanic breccia, andesite, limestone), indicating shallow bedrock and low amplification potential. In contrast, stations such as TL01, TL05, TL07, and TL08 located on Holocene alluvium and coral reef sediments exhibit low f₀ (40), and Kg values (>900), suggesting soft sedi- ments with significant seismic amplification risk. These areas also coincide with high shallow earthquake activity, reinforcing their classification as seismic hazard- prone zones. A positive correlation is observed between low Vs₃₀, high soil vulner- ability index (Kg), and the characteristics of surface geological formations. This study confirms that integrating Horizontal to Vertical Spectral Ratio (HVSR) anal- ysis with geological, Vs₃₀, and seismic data offers a reliable approach for microzo- nation and seismic risk mapping in tectonically complex regions. The results in the Nusa Tenggara Timur (NTT) region, including Flores, Lembata, Alor, and the sur- rounding areas such as Timor Leste, provide critical input for prioritizing seismic risk mitigation, guiding site-specific engineering design, and supporting resilient urban planning across the region.