## 5 Conclusions  
In this research, we performed travel time tomographic inversion using an initial 3D velocity model. These are obtained by manual re-picking of the same number of P- and S-arrivals and simultaneous determination of the 3D structure for the \(\rm{Vp}\), \(\rm{Vs}\), and \(\rm{Vp/Vs}\) ratio, as well as relocating the hypocenter location. The results show a high \(\rm{Vp}\), high \(\rm{Vs}\), and low-\(\rm{Vp/Vs}\) anomaly beneath the Minahasa Peninsula, Sulawesi's north arm, with the Celebes Sea (cross-section A-A\({}^{\prime}\), south-north) dipping to the north at a ~40\({}^{\circ}\) angle. Beneath the Celebes Sea, Sangihe Island, Molucca Sea, the Morotai Basin (cross-section C-C\({}^{\prime}\), west-east), and Sulawesi's north arm, the Molucca Sea, and Halmahera Island (cross-section D-D\({}^{\prime}\), west-east) can be interpreted as a double subduction of the Molucca Sea Plate. The tomographic image beneath the southern part of the Halmahera volcanic arc (Bacan Island) shows low \(\rm{Vp}\), low \(\rm{Vs}\), and high \(\rm{Vp/Vs}\), which indicates a possible magma source; although the volcano has been dormant until the present. The tomographic image beneath the group of volcanoes in the northern arm of Halmahera also shows low \(\rm{Vp}\), low \(\rm{Vs}\), and high \(\rm{Vp/Vs}\), which can be interpreted as a possible magma source. Differing from volcanoes in the south, however, some volcanoes in this area have erupted and are emitting volcanic ash. Both the volcanic groups on Bacan Island and the northern arm of Halmahera are connected by partial melting, as indicated by low Vp, low Vs, high Vp/Vs above the Molucca Sea Plate, which subducts to the east. The upper curvature of the Molucca Sea Plate in the northern part is submerged deeper into the upper mantle layer at a depth of \(\sim\)150 km compared with the southern part. It was also found that beneath the Central Ridge of the Molucca Sea, at latitude 1.10 \({}^{\circ}\)N, the low Vp, low Vs, and high Vp/Vs in the crust and mantle can be interpreted as highly fractured and water-saturated rock material; while high Vp, high Vs, and low Vp/Vs are observed in the western (beneath the ST) and eastern parts (beneath the HT), which can be interpreted as rock material that experienced micro-cracks that have since closed due to high pressure. Further north, the low Vp, low Vs, and high Vp/Vs anomalies shift eastward and extend to HT with a clear boundary between low and high velocity below the Central Ridge of the Molucca Sea. The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app122010520/sl](https://www.mdpi.com/article/10.3390/app122010520/sl)]([https://www.mdpi.com/article/10.3390/app122010520/sl](https://www.mdpi.com/article/10.3390/app122010520/sl)), Figure S1: Molucca Sea Plate profile overlaid with seismic profile from 1959-1966 which forms a double subduction pattern was modified from [7]. NW: Northwest, SE: Southeast. Sign? : the length of the plate subduction to the west and east is not yet known. The black dot denotes earthquakes. ; Figure S2: Histogram of the number of phases per station; Figure S3: Horizontal and vertical ray path coverage (West-East: below and North-South: right). The brown line is the ray path connecting hypocenters locations with BMKG stations; Figure S4: Wadati diagram for all events used in this study. Tp is arrival times of the P-wave and Ts\(-\)Tp is the difference between S- and P-waves arrival times. The Gray dot is the picks of phase time from each station at a certain event, and the solid line is the slope to determine the Vp/Vs value which is 1.76; Figure S5: Trade-off curve between data variance and model variance. We chose damping parameters Vp and Vp/Vs to be 150 and 130, respectively.