Koya earthquake

38 km east of Nobeoka lies offshore in the Kyushu Trench, the active convergent boundary where the Philippine Sea Plate is being thrust beneath the Eurasian (Amurian) Plate. This segment of the Nankai‑Kyushu subduction zone hosts an intense seismic environment, with slip rates estimated at roughly 10 mm yr⁻¹ and a history of large thrust events—including the Mw 6.2–6.3 earthquakes of 2016 that caused widespread damage on the Kyushu coast. The trench and its accretionary prism are characterized by frequent moderate‑to‑large‑magnitude earthquakes, and the region is considered a key part of the broader Nankai‑Trough system that can generate megathrust ruptures. The tectonic regime here involves the interaction of the Philippine Sea Plate, the Amurian microplate, and adjoining East China Sea plates, creating a complex stress field that contributes to both offshore thrust and shallow shallow‑depth earthquakes along the coast.