**1. Introduction**

The Molucca Sea region is a very complex area where three main plates, the Eurasian, Australian, and Pacific plates, interact and create a collision zone known as the Molucca Sea Collision Zone [1–5]. The activity of these plates and the subduction of the Molucca Sea Plate are the main factors causing the high level of seismicity in this area. Tectonic activity is also increased by the Molucca Sea Plate which completely submerges the Halmahera microplate [6] and the Sangihe microplate [7], as shown in Figure S1.

The Molucca Sea Collision Zone is located between the Halmahera microplate in the east and the Sangihe microplate in the west with the Molucca Sea in the center. One

**Citation:** Rachman, G.; Santosa, B.J.; Nugraha, A.D.; Rohadi, S.; Rosalia, S.; Zulfakriza, Z.; Sungkono, S.; Sahara, D.P.; Muttaqy, F.; Supendi, P.; et al. Seismic Structure Beneath the Molucca Sea Collision Zone from Travel Time Tomography Based on Local and Regional BMKG Networks. *Appl. Sci.* **2022**, *12*, 10520. https:// doi.org/10.3390/app122010520

Academic Editors: Guofeng Liu, Zhifu Zhang and Xiaohong Meng

Received: 20 September 2022 Accepted: 14 October 2022 Published: 18 October 2022

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collision resulted in the formation of a high ridge in the middle of the two microplates, known as the Mayu-Talaud Ridge, which is characterized by intense, shallow earthquakes and an low gravity anomaly [3,5,7,8]. The collision is ~250 km apart [4] and convergently collides with the Molucca Sea Plate [4,9] at a movement of 1.5 cm/yr [9]. The Molucca Sea Plate has an inverted U- or V-shape [2,8,10–13] and has slab subducting to the west under the Sangihe Arc and subducts to the east under the Halmahera Arc. The subduction of the Molucca Sea Plate in both eastern and western directions makes for arc volcanisms in the Halmahera and Sangihe Arcs that run parallel to the subduction zone [8,13,14] with its two Wadati–Benioff Zone active of mantle earthquakes [4,7,10]. The collision of the Halmahera and Sangihe Arcs with the subduction of the Molucca Sea Plate remains active today [15–18].

The Molucca Sea is one of the most seismically active areas in the world [19], as shown in Figure 1. There are many seismic zones in this area, such as the seismic zone concentrated in the central Molucca Sea at a depth of <60 km beneath the Mayu-Talaud Ridge, the seismic zone beneath the Sangihe and Halmahera Arcs, the seismic zone around the Gorontalo Basin, along the Minahasa peninsula, around Morotai Island, the Sulawesi Sea, the Molucca-Sorong Fault, the Bacan-Sorong Fault, the Sula-Sorong Fault, the Sulawesi slab in the northern part of Sulawesi's north arm, and the area around the Philippine slab in the eastern part of Halmahera Island. The red dots in Figure 1 indicate shallow earthquakes; these are mostly concentrated in the central Molucca Sea area beneath the Mayu-Talaud Ridge and the Gorontalo Basin; the brown dots indicate intermediate earthquakes that are mostly concentrated beneath the Sangihe and the Halmahera Arcs; the yellowish blue dots indicate deep earthquakes that are concentrated in the Sulawesi Sea area.

**Figure 1.** The horizontal distribution of seismic map of the research location around the Molucca Collision Zone. SLW: Sulawesi island; ML: Molucca island; PA: Papua island; HLM: Halmahera island; SNA: Sulawesi's north arm; SS: Sulawesi slab; PS: Philippine slab; HT and ST: Halmahera and Sangihe Trench; HA and SA: Halmahera and Sangihe volcanoes arc; CS: Celebes sea; MP: Minahassa peninsula; GB: Gorontalo Basin; MS: Molucca Sea; MTR: Mayu-Talaud Ridge; BSF, MSF, and SSF: Bacan, Molucca, and Sula Sorong Faults, respectively.

Numerous studies on seismicity that relate to the structure and geometry of the Molucca Sea Plate have been carried out by previous researchers. One such study covers the two Benioff zones that lie in opposite directions and form a double subduction pattern in the Molucca Sea Plate [5,7,8,10,13]. The subducted slab under the Sangihe Arc can be identified as far down as the mantle transition zone layer at a depth of ~650 km; the subducted slab under the Halmahera Arc reaches the solid upper asthenosphere layer at a depth of ~250 km [8]. According to Cardwell et al. [10], the subducting slab to the east under the Halmahera Arc subducts at an angle of ~45◦ and reaches a depth of ~230 km; Hall and Spakman [20] show that it reaches depths of ~400 km, which is in line with recent research [21] that used 3D earthquake data distribution plots from ISC-EHB to identify slab configurations that were not modeled by slab2. Hutchings and Mooney [12] showed that the subducted slab in the Sangihe Arc at depths of ~250 to 350 km has a seismic gap, while the tomography image of the slab appears to be continuous.

Tomographic studies have been carried out by previous researchers, including Puspito et al. [22] who used P-phase travel time data from ISC bulletins; their work shows a high-velocity zone that forms a double subduction of the Molucca Sea Plate. The slab is subducted to the west under the Sangihe Arc and reaches to the lower mantle. Meanwhile, the slab that subducts to the east under the Halmahera Arc reaches a depth of 400 km, which is conformable with the results of the study by Widiyantoro and Hilst [23]. This research was later updated by Widiyantoro [24] who shows that the subducted slab to the west reaches the lower mantle with a folded-looking slab. Huang et al. [25] also detected a high-velocity zone in the form of bipolar subduction. Meanwhile, Zhang et al. [26] showed that the Molucca Sea Plate has a positive velocity anomaly with unique asymmetry. Fan and Zhao [27,28] showed that the subduction of the Molucca Sea Plate ~5 Ma ago propagated from south to north; they were able to detect the Molucca Sea Plate using anisotropic tomography. Zenonos et al. [29], using data from ISC-EHB, show the high-velocity Vp and Vs of the double subduction of the Molucca Sea Plate in which the high Vp reached a depth of ~700 km beneath the Sangihe Arc and ~400 km beneath the Halmahera Arc. The high Vs reached a depth of ~500 km, which is in positive agreement with a study performed by Chen et al. [21] who used only P-wave data. Zenonos et al. [30] showed that negative Vp/Vs anomalies can reach ~160 km, and positive Vp/Vs anomalies can reach depth of ~200 km. Another study by Cao et al. [31] observed the mantle flow using seismic anisotropy. Their study shows that beneath the Halmahera Arc the sub-slab mantle flow is oriented oblique to the Halmahera trench, while the sub-slab mantle flow beneath the Sangihe Arc runs in a parallel direction to the Sangihe trench.

According to previous research, tomography is generally based on data from global teleseismic earthquakes within a broad region, producing a tomographic image with a broad resolution that displays only relatively large objects. Tomography around the Molucca Sea region usually only shows the main features of the Molucca Sea Plate [22,23,26,29]. Several local researchers in the Molucca Sea area have used different methods [2,17,18]. However, their results extend only up to the layer of the Earth's crust; the tomographic results have not shown anomalies of 3D velocity structures due to volcanic activity in the subduction zone. The research undertaken in this study made use of local and regional data from BMKG (Figure 1) to obtain more detailed features of the Molucca Sea region, especially the estimated subduction angle of the Molucca Sea Plate from south to north beneath Sulawesi's north arm; the relation between the subduction zone of the Molucca Sea Plate subducts to the east and its volcanic activity in the Halmahera Volcanics Arc, and the depth of submerged of the double subduction of the Molucca Sea Plate in the south and north.
