Introduction to the 3-D seismic observation of the north Sulawesi subduction zone and the study of initiation mechanism

The Celebes sea has a unique evolutionary history. When the oceanic crust of Celebes below the north Borneo and Sulu arcs stopped subducting, the plate on the other side began to subduct towards the northern arm of the Sulawesi Island. Several unique geological structures, such as the subduction transform edge propagator, double inward subduction were formed. This dynamic tectonic process, from subduction cessation to initiation in a relatively short period of time is a distinctive feature of the western Pacific subduction system. To explain its physical mechanisms of the process, many models to explain the subduction initiation mechanism have been proposed, such as the post-collision development from foreland arcs, gravity instability initialization at the arc/continental margins and sub-lithospheric mantle convection etc.. Due to deficiency of high resolution seismological data of the incipient subduction zone structure, these mantle dynamic models are still remaining in debate. Acquiring earthquake observation data from the incipient subduction system is not only the basis for studying the deep structure and evolution of its lithosphere, but also a key element in understanding the initial mechanism of the subduction process. This paper introduces the 3-D offshore earthquake observation experiment focusing on the structure of the incipient subduction zone of the North Sulawesi, which is supported by the National Science Foundation of China key project " 3-D seismic observation and plate movement mechanism of the inward subduction system in Celebes Sea and proto-South China Sea”. The objective of this experiment is to reveal the deep structure of the incipient subduction system and the mechanism of plate movement, 27 broadband ocean bottom seismometers (OBS) were deployed from August 15 to 25, 2019 through international collaboration to record seismic data for 10 months. It plans to study the 3-D structure of the lithosphere of the north Sulaweis subduction zone and carries out numerical simulation analysis of its plate movement. Using the OBS and the land seismic stations data together, we can study the deformation pattern and characteristics of subducting plates, overlying crust and its relationship with the continuous shorting of double subduction, to reveal the mechanism of this incipient subduction system.