Improved Aftershock of the 2006 Yogyakarta MW 6.4 Earthquake Sequence Using Deep Learning and Its Correlates with Tectonic Setting

The 2006 Yogyakarta MW ~6.4 earthquake epicenter was located near the Opak Fault, but slightly farther east, with aftershocks also spreading more toward the east of the fault. The distribution of aftershock did not align precisely with the Opak Fault, raising questions about whether the mainshock originated from the Opak Fault or from another nearby fault. One hypothesis suggests that an eastward dipping fault caused the eastern distribution of the aftershocks. However, the previous studies by Saputra et al. (2021) and Ramdhan et al. (2025a, b) concluded that the MW ~6.4 earthquake had a westward dipping fault, located to the east of the Opak Fault. In this study, we applied a deep learning method to analyze the arrival times of P and S waves, providing more consistent results than previous approaches. This method has not previously been applied to the analysis of the Yogyakarta MW ~6.4 aftershock sequence. We used the arrival times of P and S waves to determine the locations and local magnitudes (ML) of aftershocks. First, we used the grid‑search method to determine the absolute hypocenter locations. Then, we updated the velocity model and relocated the events to better represent the seismic conditions in the region. Finally, we selected events to refine the distribution pattern and understand the tectonic setting around the mainshock. The results show that aftershocks in the eastern part of the study area occurred at greater depths compared to those on the western side. To further understand this pattern, we calculated Coulomb stress changes using the focal mechanism from the Global CMT, which aligns with the USGS catalog. The analysis reveals that the shallower earthquakes on the western side of the fault correlate with areas of positive Coulomb stress change. These findings suggest that the 2006 Yogyakarta earthquake was likely triggered by a westward‑dipping fault associated with the Ngalang Fault.