Geophysical assessment for earthquake mitigation: probabilistic seismic hazard analysis and Earth gravity inversion in the Sorong fault zone, West Papua, Indonesia

This study integrated seismological and gravity analyses to understand the tectonic characteristics and seismic hazard potential of the Sorong fault zone in Papua, Indonesia. Seismic data map hypocentre distribution, analyse seismicity, and calculate peak ground acceleration (PGA) and spectral acceleration (Sa) within a probabilistic seismic hazard analysis (PSHA). Bouguer gravity data were analysed through regional-residual anomaly separation, two-dimensional and three-dimensional (3D) Euler deconvolution (ED), and 3D gravity inversion to estimate the subsurface density distribution and fault geometry. The seismicity parameter analysis indicated a magnitude of completeness (Mc) of 4.5 and a b-value of 0.93±0.29, reflecting a balance between large and small earthquakes and indicating high stress in the region. The PSHA based on the Sorong fault source in the Dampier segment showed PGA values ranging from 0.016 to 0.265 g for a 50-year return period. The Sa for periods of 0.2, 0.3, 0.6, 1.0, and 2.0 s shows maximum values for short periods, and vice versa. These results are consistent with the regional and global PSHA analyses for Papua Island. Gravity data analysis showed spectral differences between shallow and deep sources, suggesting a clear differentiation in the rheology of the deep and shallow crust; therefore, the two zones exhibit different mechanical behaviours. The ED identifies clusters of solutions potentially associated with the Sorong fault segments despite the interpretative limitations inherent to the method. The 3D gravity inversion suggests the presence of high-density zones interpreted as magmatic intrusions and low-density zones related to shear zones or deformed rocks, pending further validation of this interpretation. A comparison suggests a possible spatial relationship between the density anomaly zones and the concentrations of seismic activity, which may reflect the structural configuration of active faults. These findings enhance our understanding of tectonic mechanisms and contribute to seismic hazard assessments of the Sorong fault zone.