*4.3. Double Subduction of the Molucca Sea Plate*

The double subduction zone of the Molucca Sea Plate is shown in the vertical crosssection of C-C (west–east), crossing the Celebes Sea, Sangihe Island, Molucca Sea, and the Morotai Basin (Figure 8); and the vertical cross-section D-D (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 ~300 km. Based on the resolution test (Figures S6–S8), the subduction pattern is shown to reach a depth of ~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 ~210 km. High-Vp, high-Vs, and low-Vp/Vs anomalies subducting eastward to a depth of ~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 Chou et al. [50], Zhao [46], and Zenonos 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 [9,22–24,27,60] in which the plate subducting to the west can reach the mantle transition layer at a depth of ~600 km. According to Fan and Zhao [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 ~150 km. Based on the geometry model of slab2 (green line) from Hayes 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 [30,61–63]. 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 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 ~300 and ~250 km, respectively. 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 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 Chou et al. [50], Zhao [46] and Zenonos et al. [29,30]; i.e., high Vp, high Vs, and low 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 [9,22–24,27,60]. 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 Hayes et al. [45]. Fan and Zhao [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 Puspito 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-Vp, high-Vs, and low-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 [30,61–63]. 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-Vp, low-Vs anomalies, and high 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'), 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-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.
