Shallow tremors in the northern sulawesi subduction zone

Tectonic tremors (TTs), composed of a swarm of low-frequency earthquakes (LFEs), constitute a type of slow earthquakes characterized by a lack of high-frequency energy. Previous studies have suggested that slow earthquakes, which usually occur near the megathrust earthquake rupture zones, are crucial for deepening our understanding of seismic activity. The Northern Sulawesi subduction zone (NSSZ) is situated at the convergence of the Eurasian, Australian, and Philippine Sea plates, experiencing frequent earthquakes that may trigger local tsunamis due to the complex tectonic setting. Until now, the lack of shallow observations has limited the understanding of the shallow tectonic structure beneath the NSSZ. We observe episodic shallow TTs using 8 Ocean Bottom Seismometers (OBS) deployed near the NSSZ, indicating the presence of stable sliding near the subduction boundary. Our research results reveal that the locations of shallow TTs align with the boundary of the weakly coupled plate interface where the relative Coulomb stress is weaker. Additionally, we discover that the sedimentary environment in the shallow subduction zone and the dehydration of serpentinite within the plates provide favorable conditions for high pore fluid pressure, thereby promoting the occurrence of shallow TTs. Furthermore, we try to establish a connection between deep earthquakes and shallow TTs, exploring the possibility of deep-seated stress propagating from the deep crust to the shallow seismic zone through faults or plate boundaries.