Comprehensive geophysical and petrographic approach in seismic hazard assessment: case study of Ransiki Fault Area, South Manokwari Regency, Eastern Indonesia

Abstract

The Ransiki fault zone in South Manokwari Regency, West Papua, Indonesia, poses significant seismic hazards to residents and infrastructure in the region. This study assessed these hazards using a multidisciplinary approach that integrated Probabilistic Seismic Hazard Analysis (PSHA), Horizontal-to-Vertical Spectral Ratio (HVSR) analysis from microtremor recordings, Vertical Electrical Sounding (VES), and petrographic analysis. The PSHA was conducted at 72 points (node spacing 0.1°, ~ 11 km) with 18 HVSR and VES control points using a logic tree with three ground motion prediction equations (GMPE). The results show spectral acceleration (S_a) of 0.049–1.185 g at T = 0.2 s and 0.006–0.129 g at T = 2.0 s for a 10% Probability of Exceedance (PoE) in 50 years (~ 475-year recurrence), with the highest hazard along the Ransiki Fault. HVSR analysis at 18 stations revealed natural frequencies (\(f_{0}\)) of 0.350–9.480 Hz, amplification factors (\(A_{0}\)) of 1.970–5.460, and dominant periods (\(T_{0}\)) of 0.105–2.860 s. HVSR curves from Raipawi Village (\(f_{0}\) = 0.35 Hz; \(A_{0}\) = 5.46) and Wariab Village (\(f_{0}\) = 1.5 Hz; \(A_{0}\) = 4.70) show site response contrasts that reflect subsurface geological heterogeneity. VES surveys at four earthquake-affected locations identified water-saturated sediment layers and potential aquifer zones at depths of 5–60 m, with resistivity values of 2.62–2554.73 Ohm m, indicating high liquefaction risk in the southern coastal plains. Petrographic analysis identified andesite and sublitharenite as the dominant lithologies, each with different seismic loading susceptibility. The integrated dataset produced a detailed seismic hazard zoning map that established a vulnerability gradient from north to south. This study is the first geophysical and petrographic seismic hazard assessment in the Ransiki District, providing a scientific foundation for earthquake-resistant infrastructure design and spatial planning in high-risk Eastern Indonesia.