## 4 Discussion
### Double Subduction of the Molucca Sea Plate  
The double subduction zone of the Molucca Sea Plate is shown in the vertical cross-section of C-C\({}^{\prime}\) (west-east), crossing the Celebes Sea, Sangihe Island, Molucca Sea, and the Morotai Basin (Figure 8); and the vertical cross-section D-D\({}^{\prime}\) (west-east), crossing Sulawesi's north arm, the Molucca Sea, and Halmahera Island (Figure 9). Based on tomographic inversion results in the C1 area, the high-Vp, high-Vs anomalies subduct to the west and reach the asthenosphere layer at a depth of \(\sim\)300 km. Based on the resolution test (Figures S6-S8), the subduction pattern is shown to reach a depth of \(\sim\)400 km (CRT result: not good resolved areas), while a low Vp/Vs is seen in the partially molten asthenosphere layer at a depth of \(\sim\)210 km. High-Vp, high-Vs, and low-Vp/Vs anomalies subducting eastward to a depth of \(\sim\)200 km are also observed. This feature can be interpreted as a double subduction of the Molucca Sea Plate, which subducts westward beneath the Sangihe Islands and eastward beneath the Morotai Basin. This is also followed by the presence of intermediate and deep earthquakes in the upper mantle layer that forms a double subduction slab pattern, as indicated by high seismic velocity [53]. These results are consistent with those of [PERSON] et al. [50], [PERSON] [46], and [PERSON] et al. [29; 30] who suggest that high Vp, high Vs, and low Vp/Vs can be associated with the feature of subducted slab. Previous studies of P-wave tomography also identified a double subduction of the Molucca Sea Plate [9; 22; 23; 24; 27; 60] in which the plate subducting to the west can reach the mantle transition layer at a depth of \(\sim\)600 km. According to [PERSON] and [PERSON] [27], the Molucca Sea Plate is submerged deeper in the north than in the south. These studies also indicate that the arch of double subduction of the Molucca Sea Plate has submerged into the upper mantle at a depth of \(\sim\)150 km. Based on the geometry model of slab2 (green line) from [PERSON] et al. [45], the arch of the double subduction is closer to the surface in the uppermost mantle layer. The C2 area is located beneath the Sangihe Trench (ST), which is a collision zone of the Sangihe continental crust that moves eastward with an overriding plate structure [26] and the Molucca Sea Plate that subducts to the west. The tomographic inversion results show that high-Vp, high-Vs, and low-Vp/Vs anomalies in the mantle crust can be interpreted as rock material that experienced micro-cracks that have since closed due to high pressure [30; 61; 62; 63]. The C3 area is located beneath the Halmahera Trench (HT), which is a collision zone of the Halmahera continental crust that moves westward, having an overriding plate structure [26] with the Molucca Sea Plate subducting to the east. The tomographic inversion results show that low-Vp, low-Vs, and high-Vp/Vs anomalies in the crust and mantle can be interpreted as highly fractured, water-saturated rock material [63]. Our results obtained beneath the Central Ridge of Molucca Sea, indicate a clear presence of boundary between ST which has high-velocity anomalies and low Vp/Vs and HT, which has low-velocity anomalies and high Vp/Vs. This boundary also has a signature of high seismicity area. An earthquake of magnitude 6.2 occurred on 10 July 2021 on this boundary that can be interpreted as a collision zone between fault planes or fractures [10]. The boundary may represents material with different densities between the western and eastern parts of the Central Ridge of the Molucca Sea which have upraised ophiolite bodies [17; 18]. In the vertical cross-section D-D\({}^{\prime}\) of area D1 (Figure 9), the results of the inversion tomography show the presence of a double subduction of the Molucca Sea Plate indicated by high-Vp, high-Vs anomalies. It subducts westward and eastward, reaching a solid asthenosphere layer at a depth of \(\sim\)300 and \(\sim\)250 km, respectively.