New Insight on the Active Fault System in the Halmahera Volcanic Arc, Indonesia, Derived from the 2022 Tobelo Earthquakes

On 9 January 2022, two shallow earthquakes occurred in the east arm of Halmahera, Indonesia, with magnitudes of Mw 5.2 and 5.4 felt at III-V modified Mercalli intensity. The existence of the causative fault for the earthquakes was unknown. One possible explanation is that the earthquakes were caused by the activity of an inactive fault striking in the southeast-northwest direction, and possibly linked to the volcanic activities in Halmahera. Thus, further analysis of their characteristics is crucial to identify the previously an inactive fault. In this study, we relocated the associated earthquake hypocenters using the double-difference method with an updated velocity model, and determined the focal mechanisms by employing full waveform inversion using moment tensor analysis. The focal parameters were used to analyze the static stress changes. The relocated hypocenters show specific hypocenter clusters, aligning in the north- south direction, associated with left-lateral faulting mechanisms. This north-south alignment differs from the northeast-southwest trend of some other active faults in Halmahera. We propose that the newly found Tobelo fault dips at an angle of around 80 degrees to the west. The seismic stress change from the mainshock and the foreshock of the Tobelo earthquakes corresponds with the distribution of the aftershocks. A detailed analysis of unidentified faults in the high seismicity area is essential to comprehend the tectonic study of Halmahera Island and its seismic hazard assessments.