**3. Results**

Based on the re-picking phase and distribution of available BMKG stations, we determined the location of the hypocenter using the NonLinLoc Global mode. The estimated mean error in x, y, and z directions using the covariances values are 10,198 km, 14,081 km, and 19,035 km, respectively. The distribution frequency estimated error location in the x, y, and z directions is shown in Figure S9. The distribution between the number of events and the azimuthal gap is shown in Figure S10, and the epicentral distance distribution is shown in Figure S11. We limit the value of travel time residuals in the range of −10 ≤ initial travel time residuals (itr) ≤ 10 to control the quality of the input data for the tomographic inversion process. The bad picks can be traced in unit 20 of simulps output file, as shown in Figure S12. Before the inversion, the initial travel time residual values varied in the range of −9.993 s to 9.858 s, with a mean value of −1.111 s. After inversion, the final travel time residuals varied in the range of −7.148 to 6.197 s, with a mean value of 0.156 s, forming a Gaussian distribution centered around 0 s. Tomographic inversion achieved the convergence after the 15th iteration with the weighted Root Mean Square (RMS) residual reduced by 47%, initially from 0.82312 s down to 0.43871 s (Figure S13). The RMS residual before the inversion ranged from 0.06 to 1.26 s with an average value of 0.54 s. After inversion, the range of values was down to 0.04 to 0.83 s with a mean value of 0.36 s (Figure S14) or reduced by about 33%.

The Interpretation of the tomogram becomes valid and has a high confidence level by performing a resolution test of the seismic tomography model. Therefore, we present the CRT results shown in Figure 5 for Vp and Vp/Vs of profiles A-A , B-B , C-C , and D-D and Figure S15 horizontal section for Vp and Vp/Vs with the recovered area is delineated by dashed purple lines. The CRT positive recovery for A-A is beneath the Gorontalo Basin, Sulawesi's north arm, and the Celebes Sea up to a depth of 210 km. The CRT positive recovery for the B-B profile is beneath the Halmahera Volcanic Arc up to a depth of 240 km; for profile C-C the CRT positive recovery is shown beneath the Celebes and Molucca Seas, and the Morotai Basin up to a depth of 210 km. For profile D-D , the CRT positive recovery is beneath Sulawesi's north arm, the Molucca Sea, and Halmahera Island up to a depth of 300 km. These results are consistent with the density distribution of P- and S-rays as revealed by the DWS, RHC, and DRE results (Shown in Figures S6–S8); the highest Vp and Vp/Vs values are listed in Table S1. The primary resolution test we used to interpret the tomographic results was the CRT, supported by the DWS, RHC, and DRE results. Hence, this research focused on the travel time tomography results in four areas in the subduction zone beneath Sulawesi's north arm, the subduction zone beneath the Halmahera Volcano Arc, and the double subduction zone of the Molucca Sea.

**Figure 5.** *Cont.*

**Figure 5.** The vertical cross-section of CRT profiles. Profile A-A (south–north), B-B (south–north), C-C (west–east), and D-D (west–east) are vertical cross-sections of the CRT recovery model from the inversion process. The purple dashed line is the positive CRT recovery area for tomographic interpretation. The first column is Vp perturbation, the second column is Vp/Vs perturbation, and the red triangles signify volcanoes.

Tomographic inversion results of cross-section A-A , as shown in Figure 6 as follows:

**Figure 6.** Vertical cross-section of A-A (south–north) of the Molucca Sea Plate, crossing the Minahasa Peninsula, Sulawesi's north arm, and the Celebes Sea. The red color indicates low velocity and low Vp/Vs ratio, and the blue color indicates high velocity and high Vp/Vs ratio. The A1 and A2 are areas for the interpretation of tomographic inversion results. The black dots signify earthquakes within a radius of 30 km ≤ A-A ≤ 30 km, and the yellow stars are significant earthquakes. The green line and the green three-dimensional surface plot are the slab2 models of the Molucca Sea Plate. The yellow line is the slab2 model of Sulawesi [45]. NST is the North Sulawesi Trench. The red triangles signify volcanoes. The blurred area is an area that is not well-resolved.

Tomographic inversion results of cross-section B-B', as shown in Figure 7 as follows:

**Figure 7.** Vertical cross-section of B-B' (south–north) crossing beneath the Halmahera Volcanic Arc. The red color indicates low velocity and low Vp/Vs ratio. The blue color indicates high velocity and high Vp/Vs ratio. The B1, B2, and B3 are areas for the interpretation of tomographic inversion results. The green line and the green three-dimensional surface plot are the slab2 models of the Molucca Sea Plate [45]. The black dots signify earthquakes within a radius of 30 km ≤ B-B ≤ 30 km. SeT: Seram Trench; HT: Halmahera Trench; Bi: Bibinoi Volcano; Bs: Batusibela Volcano; Am: Amasing Volcano; Br: Buku-Rica Volcano; Rr: Rogi-Rogi Volcano; Ja: Jailolo Volcano; To: Todoko-Ranu Volcano; Ga: Gamkonora Volcano; Ib: Ibu Volcano; Tb: Tobaru Volcano. The blurred area is an area that is not well-resolved.

Tomographic inversion results of cross-section C-C , as shown in Figure 8 as follows:

**Figure 8.** Vertical cross-section of C-C (west–east) crossing the Celebes Sea, Sangihe Island, Molucca Sea, and Morotai Basin. The red color indicates low velocity and low Vp/Vs ratio. The blue color indicates high velocity and high Vp/Vs ratio. The C1, C2, and C3 are areas for the interpretation of tomographic inversion results. The black dots signify earthquakes within a radius of 30 km ≤ C-C ≤ 30 km, and the yellow star is a significant earthquake. The green line and the green three-dimensional surface plot are the slab2 models of the Molucca Sea Plate, and the brown line is the slab2 model of the Philippines [45]. ST: Sangihe Trench; HT: Halmahera Trench; the blurred area is an area that is not well-resolved.

Tomographic inversion results of cross-section D-D , as shown in Figure 9 as follows:

**Figure 9.** Vertical cross-section of D-D (west–east) crossing Sulawesi's north arm, the Molucca Sea, and Halmahera Island. The red color indicates low-velocity areas and low Vp/Vs ratio, and the blue color indicates high-velocity areas and high Vp/Vs ratio. The D1, D2, D3, D4, and D5 are areas for the interpretation of tomographic inversion results. The black dots signify earthquakes within a radius of 30 km ≤ D-D ≤ 30 km, and the yellow star is the magnitude of a significant earthquake. The green line and the green three-dimensional surface plot are the slab2 models of the Molucca Sea Plate; the yellow line is the slab2 model of Sulawesi [45]. Thus, So: Soputan Volcano; ST: Sangihe Trench; the reserve arrow is Tifore island; HT: Halmahera Trench; Ja: Jailolo Volcano; To: Todoko-Ranu Volcano; Wa: Wato-Wato Volcano; the blurred area is an area that is not well-resolved.

Three-dimensional tomographic inversion results combine cross-section profile A-A and profile D-D , as shown in Figure 10 as follows:

**Figure 10.** The 3D visualization of the Molucca Sea Plate, combining vertical cross-section profile A-A (south–north) and profile D-D' (west–east) from the P-wave tomography inversion results. The earthquake events follow the high Vp anomaly pattern. The red color indicates low-velocity areas and low Vp/Vs ratio, and the blue color indicates high velocity and high Vp/Vs ratio. The white dots signify earthquakes; the yellow stars signify earthquakes with significant magnitudes. The red triangle is a volcanic arc located in Sangihe and Halmahera.
