Hypocenter Relocation of Aftershocks in Yogyakarta June–August 2006 Using the Double-Difference Method

Abstract

The 26 May 2006 Yogyakarta earthquake (Mw 6.3) generated a significant aftershock se-quence that persisted for several months, reflecting complex fault interactions in the region. This study aims to refine the spatial distribution of aftershocks using a larger dataset than previous research, consisting of 10,307 automatically detected events from Sani et al. (2025), compared to 2,170 events in Librian et al. (2024). Hypocenter relocation was performed using the double-difference method with two velocity models, ak135 and Koulakov, to evaluate their effectiveness in representing the subsurface geological conditions. The results show that 7,254 events (70.4% of the dataset) were successfully relocated, with hypocenters displaying clearer clustering along the Ngalang Fault and in two NW–SE trending structures, one corresponding to the Oyo Fault and the other previously unmapped. Residual analysis indicates that the Kou-lakov model yields smaller travel-time residuals and a more coherent seismicity pattern, consistent with mapped geological structures. In contrast to earlier studies reporting pre-dominantly east-dipping seismogenic zones, our extended dataset reveals west-dipping seis-micity at depths of 10–25 km. The earthquakes generally have steep to near-vertical dips and involve ruptures along multiple branch faults, including the Opak, Ngalang, Oyo, and a parallel fault north of Oyo. Persistent activity for up to three months after the mainshock suggests the influence of fluid presence and high-temperature conditions in the crust. These findings con-tribute to an improved seismotectonic interpretation of the Yogyakarta region and highlight the need for further studies, such as velocity and attenuation tomography, to enhance seismic hazard assessment.