The 5.3 is a normal faulting earthquake (stretching), not a thrust. And, if it was a quake on the subduction zone it would not be located right at the sea floor boundary; it would be located to the south where the megathrust is deeper. They put the depth as 28 km, but I don't know if this has meaning: it would be in the oceanic lithosphere below the crust.

I think if there was to be a precursor to a Great subduction quake it would be on or near the megathrust and not like this one.
Chris