Detailed Seismic Structure Beneath the Opak Fault Zone, Yogyakarta, Indonesia: Evidence for an Interconnected Fault System and Implications for Seismic Hazard

ABSTRACT Understanding the structure of the Opak fault in Yogyakarta, Indonesia, is crucial for accurate seismic hazard assessment. Despite identifying nearby local faults, the whole structure of the Opak fault remains unclear. Our detailed study reveals that the Opak fault and its surrounding faults are interconnected within a single fault system. This new insight will improve disaster mitigation efforts in Yogyakarta. Our approach integrates tomography, focal mechanism analysis, receiver functions (RFs), and seismicity data to provide a comprehensive understanding of the shallow crustal layer beneath an active fault. We deployed a mini regional network around the fault, combined with data from InaTEWS BMKG and a GTF (the German task force for earthquake) network, totalling 114 stations. Analyzing 3079 earthquakes with P-wave phases of 16,894 and S-wave phases of 14,422 enabled us to achieve higher-resolution seismic tomography than earlier studies, leading to more precise resolution of complex fault interactions. Our results show that the Opak fault and its neighboring faults are connected at a depth of around 12 km. In addition, at a depth of about 9 km, we observed that the P-wave velocity (VP), S-wave velocity (VS), and VP/VS ratio indicate the presence of the Opak, Ngalang, and Oyo faults, corroborated by lithological mapping. Understanding these crustal features is crucial for understanding the region’s geodynamic evolution and refining seismic hazard models. However, a detailed crustal model of Yogyakarta is still lacking. In this study, we present a new map of crustal thickness and bulk VP/VS ratios derived from P-wave RFs. Crustal thickness ranges from approximately 27 km beneath Bantul and the foothills of Mount Merapi to about 36 km under the Eastern Mountains. Using the common conversion point method applied in Bantul reveals a thin crust and a low-velocity zone extending down to about 15 km, with the Moho located between 30 and 40 km and a slab extending from 90 to 100 km. The presence of parallel faults near Yogyakarta may increase the risk of earthquakes if they were to rupture altogether. The interconnected Opak, Ngalang, and Oyo faults could significantly influence the seismic hazards faced by Yogyakarta.