Identification of Ground Vulnerability Based on Kg, PGA, And GSS Assessment by Single Station Microtremor in Gombel Village, Semarang

Gombel Village, located in the southern region of Semarang City, Central Java, is one of the areas prone to land subsidence and localized slope instability. These geohazards are frequently exacerbated by anthropogenic activities, including uncontrolled development, heavy traffic, and infrastructure construction, which contribute dynamic loading to the already fragile geological setting. According to the United States Geological Survey (USGS), the last recorded earthquake in this region occurred in 1996. Although seismic activity appears relatively infrequent, the potential for seismic wave amplification due to site effects remains significant, particularly in zones underlain by thick alluvial or weathered sedimentary deposits. This study aims to assess seismic site vulnerability in Gombel Village through the analysis of Ground Shear Strain (GSS), Peak Ground Acceleration (PGA), and the Seismic Vulnerability Index (Kg). Primary data were acquired through microtremor measurements conducted using a single-station approach across 65 strategically distributed points. The Horizontal to Vertical Spectral Ratio (HVSR) method was employed to extract the dominant frequency () and amplification factor () from ambient seismic noise. Results indicate that the seismic vulnerability index (Kg) in the study area ranges from to 22.74 cm/s. PGA values are relatively stable, ranging from 5.22 to 5.24 Gal, while GSS values vary between to strain units. High seismic vulnerability zones are identified at measurement points D2, D3, E2, E3, F2, and F3, which coincide with areas undergoing active subsidence. These zones are characterized by high amplification factors () and low dominant frequencies (), indicating the presence of thick soft sediment layers that are prone to amplify ground motion and facilitate ground deformation during seismic events. The findings underscore the necessity for detailed microzonation and the incorporation of seismic risk mitigation strategies in urban planning, especially in subsiding zones with unfavorable subsurface conditions.