Aftershock sequence of the Yogyakarta earthquake 2006 (Mw ~ 6.4), Indonesia, based on analysis of hypocenter relocation, static, and dynamic stress

The earthquake striking Yogyakarta in 2006 with a magnitude Mw ~ 6.4 stands out as one of Indonesia's most devastating inland seismic events. Within a few days post-event, the German Task Force for Earthquakes (GTF) mission strategically deployed 16 seismic stations to capture aftershocks, providing crucial insights into the earthquake's characteristics. In previous investigations, these aftershocks were useful in delineating the seismic structures beneath the earthquake zone. Notably, the study successfully identified the Ngalang Fault as the mainshock source of the 2006 Yogyakarta earthquake. The Ngalang Fault's orientation aligns with that of the Opak Fault, situated approximately 10 km to the east. This study unveils the results of relocating the aftershock hypocenters recorded by the seismic network and their correlation with the static and dynamic stress generated by co-seismic activity. Using an updated 1-D velocity model, we relocated 2141 hypocenters out of 2170 earthquakes. Daily seismicity patterns reveal that aftershocks from the Yogyakarta earthquake occurred along three active faults. Apart from the Ngalang Fault, which served as the primary source of the earthquake, additional aftershocks were observed on the southern segment of the Opak Fault and the Oyo Fault, connecting the two faults from the south. The static stress distribution indicates elevated values in the Oyo Fault and the southern segment of the Opak Fault, correlating with heightened seismic activity in these regions. Using the dynamic rupture model, the transient shear stress changes generated by the mainshock raise the stress on the Oyo and Opak Faults through dynamic rupture branching and jumping mechanisms, respectively. Consequently, seismic activity in the surrounding faults was influenced by the 2006 Yogyakarta earthquake, leading to the propagation of its aftershocks towards the two faults adjacent to the mainshock.