Seismic Site Amplification of the Cross of Opak River Fault at Yogyakarta, Indonesia, Under Ground Motions Generated by the Probabilistic Seismic Hazard Analysis

This study investigates seismic site amplification along the Opak River fault in Yogyakarta, Indonesia, using ground motions generated through Probabilistic Seismic Hazard Analysis (PSHA). Four borehole locations—Andi Jaya, Masjid Btl, Sabdodadi, and Imogiri—were selected along the fault for site-specific analysis. Due to the absence of local earthquake records, synthetic ground motions were generated using a single-site PSHA approach. The bedrock-level Uniform Hazard Spectrum (UHS) was modified to represent the Maximum Considered Earthquake risk-targeted level (UHS MCEr) by incorporating the directivity factor (Dr) and the risk-targeted factor (R). The resulting UHS MCEr was used to match a real earthquake record spectrally—the 1981 Corinth event (Greece)—to generate bedrock accelerograms. These accelerograms were propagated through the soil layers at each site using DEEPSOIL for 1D site response analysis. The peak ground accelerations (PGAs) at the surface were found to be 0.347g, 0.344g, 0.341g, and 0.411g at Andi Jaya, Masjid Btl, Sabdodadi, and Imogiri, respectively, with the highest value observed at Imogiri due to its proximity to the fault. Peak-to-peak ground acceleration amplification factors were 1.66, 1.46, 1.30, and 1.49, while spectral amplification factors were 2.206, 1.650, 1.702, and 1.616, respectively. The average amplification values (1.477 for PGA and 1.794 for spectral) significantly exceed the amplification factor 1.09 specified in the current seismic code. This substantial amplification likely contributes to the high structural damage observed in areas intersected by the Opak fault.