Seismotectonics of Sulawesi, Indonesia

Sulawesi Island, located in eastern Indonesia, lies at the triple junction of the Australian, Sunda, and Philippine Sea plates. It exhibits a distinctive K-shape, with each arm having undergone independent geological evolution. Driven by the latest Miocene collision (∼5 Ma) between the Banggai-Sula microcontinent and the East Arm, the deformation is accommodated by the corresponding motion of individual blocks at various locations, resulting in specific patterns of seismic moment release. In this study, we investigate the distributions of shallow earthquakes (< 60 km) based on the ISC-EHB and BMKG catalogs, as well as the focal mechanisms based on the GCMT catalog, to study the seismotectonics of Sulawesi. The results are grouped into five regions with characteristic seismic patterns well corresponding to tectonic signatures and geodetic observations. The main findings can be summarized in two parts. First, thrust earthquakes release seismic energy due to oceanic plate subduction, including, in order of moment release, those along the North Sulawesi Trench to the north, the Makassar Strait Thrust to the west, and the Tolo and Buton Thrust to the southeast. Notably, there is a seismic gap near the center of the North Arm that may indicate potential risk for damaging earthquakes, while those of the Batui Thrust are only minor. The second part involves left-lateral strike-slip motions along the Central Sulawesi Fault System. The occurrence of the 2018 Mw 7.6 Palu earthquake not only filled the gap of seismic deficiency on the Palu-Koro fault but also triggered overall seismic activity in Sulawesi. Additionally, note the low seismicity on the segment offshore northwest Sulawesi, which may be beyond the high relative motion between the Makassar and North Sula Block.