STUDY OF LOCAL SITE EFFECTS FOR SEISMIC VULNERABILITY MITIGATION BASED ON MICROTREMOR MEASUREMENTS IN THE NYIA AIRPORT AREA

Abstract

The Yogyakarta International Airport (NYIA) is situated on the coastal plain of Kulonprogo, underlain by alluvial deposits that increase susceptibility to local site effects during earthquakes. While previous studies in this area have examined peak ground acceleration (PGA) and liquefaction potential, research specifically addressing site effects has been limited. This study aims to evaluate site effects in the NYIA region by determining dominant frequency, amplification factor, and seismic vulnerability index. Microseismic measurements were conducted at 38 locations using a portable seismograph. The Horizontal-to-Vertical Spectral Ratio (HVSR) method was applied to process the microtremor data, and the results were mapped using GIS for microzonation. The analysis indicates that dominant frequencies in the NYIA area range from 0.51 to 10.36 Hz. Higher frequencies (>6.25 Hz) occur in the northern sector, adjacent to the Sentolo and Kebobutak Formations, whereas lower frequencies (0.51–2.5 Hz) are concentrated in areas with thicker alluvial deposits. Amplification factors vary between 1.11 and 5.82, with significant amplification (>3) observed in the airport zone and the alluvial plains of the Serang River. In contrast, lower amplification values (<2.9) appear near the geological boundaries formed by the Sentolo and Kebobutak Formations. The seismic vulnerability index ranges from 0.2 to 26, with the highest values (>19) located in the Serang River alluvial plains. The airport area exhibits a moderate vulnerability index of around 5, indicating a comparatively lower likelihood of experiencing severe site effects. Overall, the findings suggest that the NYIA zone experiences less surface-wave amplification than the surrounding Serang River alluvial region.