Seismotectonics of Sulawesi, Indonesia

Abstract

In eastern Indonesia, Sulawesi Island lies at the triple junction of the Australian, Sunda, and Phillippine Sea plates. The distinctive K-shape of Sulawesi, consisting of four arms (North, South, East, and Southeast), resulted from continuous geological evolution at different stages. Consequently, each arm has a different deformation style and tectonic setting. We studied the seismotectonics of Sulawesi using the updated dataset for earthquake seismicity and focal mechanisms, including additional supporting data. We grouped Sulawesi shallow part (< 60 kilometers) into six regions, while the deep part (60-400 kilometers) related to the Celebes Sea and Sula slabs. Regarding the shallow part, we found a lack of seismicity in the northern Palu-Koro fault and near the center of the North Sulawesi Trench, although these zones have high slip rates. On the deep part, the Celebes Sea slab showed an increasing dip as depth increased, whereas the Sula slab showed a steeply dipping slab. Moreover, the western Sula slab indicated an absence of shallow depth earthquakes (60–230 kilometers). Based on our investigations, we derived the following well-founded conclusions: (1) The northern Palu-Koro fault was beyond the high relative motion of the Makassar and North Sula blocks, resulting inactive seismicity. (2) A seismic gap or aseismic slip from the Celebes Sea subduction probably produced the low seismicity level near the center of the North Sulawesi Trench. (3) The shallow inactive seismicity in the western Sula slab might be due to the upper slab detaching during the first stage collision in the Early Miocene.