*4.2. Subduction Zone Beneath Halmahera Volcano Arc*

The subduction zone beneath the Halmahera Volcano Arc is shown in the vertical cross-section of B-B (south–north) in Figure 7. The B1 area is located under three large islands in the southern part of Halmahera: Bacan, Kasiruta, and Mandioli Islands. This area is also crossed by the Bacan–Sorong Fault, which is a branch, or splay, of the Sorong Fault that passes to the east [51], and is responsible for the seismicity in this region. Based on historical records, there was a strong and destructive earthquake, magnitude 7.1, on 16 April 1963 with an intensity scale of VIII Modified Mercalli Intensity (MMI). The most recent earthquake occurred on 26 February 2020, magnitude 5.2. The earthquake shocks were felt to be quite strong and were estimated on the V MMI intensity scale. There are several volcanoes on Bacan Island, consisting of the Bibinoi Volcano in the south, and the Batusibela, Amasing, and Buku Rica (Meng) Volcanoes in the north; all of these are stratovolcanoes. The tomographic inversion results show low-Vp, low-Vs, and high-Vp/Vs anomalies, which can be interpreted as the possible source of this magmatic arc [25,52,53] as also represented by Huang et al. [25] in the same region. These volcanoes have remained dormant until the present and without any historical eruptions. However, these volcanoes may possibly erupt in the future. This possibility is supported by the flow of fluids released from partial melting (B3 area) at a depth of ~100 km above the Molucca Sea Plate that subducts to the east under Halmahera Island [54].

The B2 area shown in Figure 7 is located beneath the northern arm of Halmahera Island and belongs to the West Halmahera Regency. The vertical cross-section passes through several active volcanoes: Jailolo, Todoku-Ranu, Gamkonora, Ibu, and Tobaru. Jailolo is a stratovolcano [54] that has experienced several earthquake swarms [55,56], but whose last eruption is unknown. Young lava flows have been found on the east side of the volcano, indicating that there has been an eruption. There is also no information about the last eruption of the Todoko–Ranu Volcano, which is a caldera volcano. Gamkonora, a stratovolcano, experienced its last eruption in 2013. The Ibu Volcano, also a stratovolcano, had its last eruption in 2012. The latest update was on 16 June 2021, in the form of a volcanic ash advisory. The Tobaru Volcano, also known as the Lolodai Volcano, is a stratovolcano and its last eruption is unknown [54].

The tomographic inversion results show low-Vp, low-Vs, and high-Vp/Vs anomalies, which can be interpreted as possible sources of a magmatic arc [25,52,53]. This interpretation is also supported by the ascending flow of fluid that originated from partial melting in the B3 area at depths of ~50–70 km. The partial melting and the subducted Molucca Sea Plate (the green line in Figure 7) are shallower in this area in comparison with the B1 area. We suggest that the difference in fluid depth is probably caused by the higher activity of volcanoes in the north of Halmahera compared with volcanoes in the south. However, further and more detailed research is necessary to ascertain this.

The B3 area is located beneath the Halmahera Volcanic Arc at a depth of ~100 km and above the Molucca Sea Plate that subducts to the east. The tomographic inversion results show low-Vp, low-Vs, and high-Vp/Vs anomalies, indicating the presence of fluids that promotes partial melting. Its good agreement with the previous studies by Nakajima et al. [57] and Zhao [53], the low Vp, low Vs, and high Vp/Vs are interpreted as partial melting materials. It is observed that the upwelling process is connected with zone B1 beneath the volcanic group located on Bacan Island, and zone B2 located beneath the volcanic group on the northern arm of Halmahera. High Vp/Vs above the subduction zone can be interpreted as slab dehydration, as shown by Chou et al. [50] who stated that Vp/Vs is an effective indicator of hydration, or melting, on the mantle wedge. In studies by Lin et al. [58] and Maruyama et al. [59], hydration is indicated by low Vp, Vs, and high Vp/Vs.
