Slab morphology and associated seismic and volcanic activities beneath Sulawesi Island, Indonesia

Abstract

The Sulawesi Island, Indonesia, represents a complex deformation with multiple subducting slabs and produces frequent earthquake, volcano, and tsunami hazards. Here, we investigate lithospheric velocity structures beneath Sulawesi Island to study the subducting slabs and their associated seismicity and volcanism. P- and S-wave arrival time data are acquired from the dense seismic network of the Indonesia Tsunami Early Warning System. We collected 192,301 P-wave and 40,345 S-wave phases arrival time data from 10,421 earthquakes with 2.5 ≤ Mw ≤ 7.5 that occurred between 2017 and 2022. Earthquake data are recorded by 137 InaTEWS seismic stations which provide sufficient ray path density for the entire Sulawesi Island. There are high velocity anomalies for subducting slabs and low velocity anomalies beneath active volcanoes. These observations revealed southward subduction of the Celebes slab, northward subduction of the Banggai-Sula slab, southwest subduction of the Sangihe slab, and double subduction of the Sangihe and Halmahera arcs northeast of the Sulawesi Island. The 2018 Mw7.5 Palu earthquake occurred on the left-lateral strike-slip Palu-Koro fault, which was formed by the northward subduction of the Banggai-Sula slab with respect to the Makassar Strait. The 2018 Mw7.5 Palu earthquake sequence and the 2018 Mamasa earthquake swarm are located with low Vp and Vs anomalies, probably due to the combination of active fault activity and fluid migration.