Analysis of Soil Dynamics and Seismic Vulnerability in Kalibening District, Banjarnegara Using the HVSR Method

On April 18, 2018, an earthquake with a magnitude of 4.4 Mw struck the Kalibening district, Banjarnegara. The observations on the damage distribution caused by the earthquake showed something unique: the areas that suffered severe damage were concentrated in villages on hills. This observation may indicate a site-effect phenomenon due to local geological conditions. In this study, we carry out microtremor measurements at 30 stations. The measurement area includes areas that have suffered damage due to the 2018 earthquake and also with minimum damage. We analyze the microtremor observation to estimate the site's characteristics and map the seismic vulnerability based on predominant frequency (f0 ) and amplification factor (A0 ) using the Horizontal to vertical spectrum ratio (HVSR) method. The results show that the predominant frequency in the Kalibening district area is 0.4 - 3.7 Hz, while the amplification factor is 2.5 - 10.7. The Kalibening Basin has a low f0 (<2.5 Hz) and A0 (<4), which indicates that the Kalibening Basin has a thick and soft soil type but low-density contrast. Areas with a high degree of damage have dominant frequency values in the range of 1.5 - 2.3 Hz and amplification of 5.1 - 9.2. These results suggest that the location has a high-density contrast and thick sediment, causing significant amplification. The seismic vulnerability index (Kg) values are in the range of 3.6 - 46.74. The area around the Kalibening Basin has a low Kg value, While the damaged areas have a moderate to high seismic vulnerability index.