The Nankai Trough is the name for the oceanic trench where the Philippine Sea tectonic plate subducts below the eastern margin of the Eurasian tectonic plate, where southern Japan sits. The interface between the two plates is the subduction fault line that hosts large earthquakes in southern Japan.
I am not an expert and English is not my first language, but isn't a trough a geological structure beneath the earth from which earthquakes can be emitted (a fault line)?