*4.1. Subduction Zone Beneath Sulawesi's North Arm*

Subducting slabs are usually indicated by high-velocity anomalies, which are caused by denser material that has a lower temperature than its surroundings [25,46,47], along with the presence of intermediate and deep earthquakes [48,49]. Based on the results of the tomographic inversion, the section A-A (south–north) shows a velocity anomaly of high Vp, high Vs, and low Vp/Vs at the A1 area, down to a depth of ~210 km (Figure 6). These features are interpreted as double subduction of the Molucca Sea Plate, which subducts westward beneath Sulawesi's north arm and the Sangihe Islands with an angle of subduction from south to north, an approximate angle of ~40◦. These results are consistent with previous research performed in different locations [29,30,46,50], which show that high Vp, high Vs, and low Vp/Vs can be identified as subducted slabs. It also has positive agreement the slab model of Hayes et al. [45]. In addition, most earthquakes are concentrated in a high-velocity area, these continued in the subduction zone as far down as a depth of ~500 km due to the very active movement of the subduction plate.

Three major earthquakes with magnitude ≥ 6 are observed in this area between January 2010 and June 2021. Two of these earthquakes (23 June 2020 and 19 January 2020 events) had a depth of ~100 km and were located in the South Bolaang Mongondow Regency around the Minahasa Peninsula of Sulawesi. These earthquakes are associated with the double subduction activity of the Molucca Sea Plate, which subducts to the west. Meanwhile, one shallow earthquake with a magnitude of 6.1 is located near the Gorontalo Basin and is associated with an active fault in the sea. The results of tomography inversion and hypocenters horizontally for each depth section are shown in Figure S16.

The A2 area in Figure 6 and the D2 area in Figure 9 are located in the northern part of Sulawesi's north arm, precisely in the North Sulawesi Trench (NST) Zone. This area reveals an anomalous pattern of high Vp, high Vs, and low Vp/Vs that can be interpreted as a slab of Sulawesi that subducts to the south with the structure above being the Molucca Sea Plate. This result is consistent with previous studies conducted by Chou et al. [50], Zhao [46], and Zenonos et al. [29,30], suggesting that high Vp, high Vs, and low Vp/Vs are associated with the presence of subducted slabs. It is also consistent with Fan and Zhao [27] which identified the slab of Sulawesi from P-wave tomography and is similar to the subduction zone geometry model (yellow line) described by Hayes et al. [45]. The existence of this slab is also followed by earthquake events, even though the level of seismic activity in this cross-section is less than the earthquake activity that occurs in the A1 area of the Molucca Sea Plate zone.
