## 4 Discussion
### Double Subduction of the Molucca Sea Plate
The Molucca Sea Plate is not well-imaged as far as the Vp/Vs ratio, and there is a discontinuity in the slab that subducts westward at depths of ~100-150 km. On the other hand, the slab that subducts to the east is well-imaged, as indicated by low \(\rm{Vp/Vs}\). The seismic pattern observed at intermediate and deep depths had a high seismic velocity, which can be associated with a double subduction slab [53]. This finding is consistent with [PERSON] et al. [50], [PERSON] [46] and [PERSON] et al. [29; 30]; i.e., high \(\rm{Vp}\), high \(\rm{Vs}\), and low \(\rm{Vp/Vs}\) are interpreted as a slab subduction. Previous studies of P-wave tomography have also identified a double subduction of the Molucca Sea Plate to depths of ~600-650 km subducting to the west, and depths of ~250-400 km subducting to the east [9; 22; 23; 24; 27; 60]. The arch of double subduction in this area is in the uppermost mantle layer near the surface, which is consistent with the geometry of the slab2 model from [PERSON] et al. [45]. [PERSON] and [PERSON] [27] showed that the arch of the Molucca Sea Plate submerged shallower in the south than in the north. The D3 and D5 areas are located in ST and HT which is a collision zone between the continental crust and the denser, heavier oceanic crust. The ST area is the collision zone between the continental crust of Sulawesi's north arm located above the Molucca Sea Plate that subducts to the west. The HT area is the collision zone between the continental crust of Halmahera above the Molucca Sea Plate and subducting to the east [26]. According to [PERSON] et al. [22], low velocity dominates the Molucca Sea collision zones and volcanic zones. The results of tomography inversion in these two areas show high-\(\rm{Vp}\), high-\(\rm{Vs}\), and low-\(\rm{Vp/Vs}\) anomalies in the crust and mantle. This feature can be interpreted as rock material that experienced micro-cracks that have since closed due to high pressure [30; 61; 62; 63]. The D4 area, located beneath the island of Tifore in the Central Molucca Sea may correspond to upraised ophiolite body [17; 18] which has an active thrust fault [11; 12]. The tectonic activity in this area causes many earthquakes of both small and large magnitudes. Tomographic inversion results in the Central Molucca Sea show low-\(\rm{Vp}\), low-\(\rm{Vs}\) anomalies, and high \(\rm{Vp/Vs}\) in the crust and mantle which indicate the presence of highly fractured brittle material and water-saturated rock [63]. In the west and east (ST and HT), we suggest that the rock material is probably not water-saturated because the micro-cracks have been covered by high pressure. However, further north (Figure 8, cross-section C-C\({}^{\prime}\)), the cracked and water-saturated rock materials shift to the east (there is a clear boundary in the Central Molucca Sea) and extend to HT. Figure 10 shows a 3D visualization of the double subduction of the Molucca Sea Plate beneath Sulawesi's north arm, the Molucca Sea, and Halmahera Island based on P-wave tomography inversion results. These results show that the Molucca Sea Plate subducts to the west beneath Sulawesi's north arm. In perspective from south to north, the state of the slab field also dips north from the Minahasa Peninsula to the Celebes Sea, having numerous earthquake events that follow the high-\(\rm{Vp}\) anomaly pattern and is interpreted as a double subduction of the Molucca Sea Plate. This is consistent with the green three-dimensional surface plot of the slab2 models of the Molucca Sea Plate [45] in Figure 6.