Seismic Vulnerability Index Study of Lasem Fault Zone Using HVSR Method Based on Microtremor Data

The Lasem Fault Zone is one of the tectonically active areas in Central Java that has the potential to trigger damaging earthquakes. To identify seismic vulnerability in this area, a microzonation approach based on soil dynamic parameters is required to represent the local response to shaking. This study aims to analyze the distribution of dominant frequency (f0) and amplification factor (A0) values, as well as to calculate and map the seismic vulnerability index (Kg) in the Lasem Fault zone using the Horizontal to Vertical Spectral Ratio (HVSR) method based on microtremor data. Data were collected at 60 observation points spread across Pati and Rembang regencies. The results of data processing show that the dominant frequency (f0) value varies between 0.59 to 20.76 Hz, while amplification factor (A0) is in the range of 1.01 to 5.05. The distribution of dominant frequency (f0) and amplification factor (A0) values is strongly influenced by the surface geological conditions, where soft and thick sediments tend to produce low f0 and high amplification factor (A0). the seismic vulnerability index (Kg) values calculated from these parameters range from 0.076 to 10.322. The resulting zoning maps show that some areas have a high level of vulnerability to earthquake shaking, especially in areas with alluvial lithology and low-lying morphology.