Travel Time Tomography Beneath the Molucca Sea Collision Zone Based on Local and Regional BMKG Networks Using a 3-D Model
Authors: Rachman Gazali , Santosa Bagus Jaya , Nugraha Andri Dian , Rohadi Supriyanto , Rosalia Shindy , Zulfakriza Zulfakriza , Sungkono Sungkono , Sahara David , Supendi Pepen , Ramdhan Mohamad , Ardianto Ardianto , Muttaqy Faiz , Afif Haunan 
Published: July 2022

Abstract
This study’s research aim has been to clearly reveal the geometry of the Molucca Sea Plate subduction zone beneath Sulawesi’s north arm, the relationship between the subduction zone and volcanic activity in Halmahera, and analyze the depths of the upper arch of the double subduction of the Molucca Sea Plate. The use of travel time tomography made it possible to reconstruct subsurface images of the complex Molucca Sea Collision Zone at greater depths and in more detail. Local and regional arrival times of P- and S- wave data from the BMKG station network were used. The network consists of 32 stations; data was recorded from January 1, 2010 to December 31, 2017. Events with magnitude > 4 were used, comprising a total of 1,647 events. The same amount of P- and S-wave arrival times were repicked at each station; ~17,628 P-wave phases and ~17,628 S-wave phases were identified. Probabilistic non-linear location was used to determine the earthquake hypocenter along with the damped least square method with 3-D seismic velocity as the initial model beneath Indonesia for the travel time tomography inversion. The tomographic inversion results show that high Vp, high Vs, and low Vp/Vs beneath the Gorontalo Basin and Sulawesi’s north arm are associated with the Molucca Sea Plate, geometrically subducting to the north at a ~40° angle. Also observed were low-Vp, low-Vs, and high-Vp/Vs in the crust mantle beneath the northern and southern Halmahera Volcanic Arc (Bacan Island), which might be associated with a possible magma source. Volcanoes in the northern part have experienced eruptions, e.g., the Gamkonora and Ibu Volcanoes. Meanwhile, volcanos in the southern part are in a dormant state and their last eruptions are unknown. This latter group of volcanoes is clearly connected with partial melting (low-Vp, low-Vs, high-Vp/Vs) above the Molucca Sea Plate, which subducts to the east.

The double subduction of the Molucca Sea Plate is shown by high-Vp, high-Vs, low-Vp/Vs which subduct westward beneath Sulawesi’s north arm and the Sangihe Islands, and subducts eastward beneath Halmahera Island. The tomographic image also shows that the Molucca Sea Plate in the north is sinking deeper (at a depth of ~150 km) compared to the south (at a depth of ~50 km). Anomalies of low-Vp, low-Vs, and high-Vp/Vs in the crust mantle beneath the Central Ridge of the Molucca Sea indicate highly fractured and water-saturated rock material; while in the west and east, beneath the Sangihe Trench (ST) and Halmahera Trench (HT), the anomaly of high-Vp, high-Vs and low-Vp/Vs indicate rock material that experienced micro-cracks that have since been closed due to high pressure. Further north, the highly fractured and water-saturated rock material shifted eastward, extending to the HT with a clear boundary in the Central Molucca Sea.