SUMMARY Intermediate-depth earthquakes occur at temperatures and pressures thought to prohibit brittle fracture and are deficient in aftershocks compared to shallow earthquakes. We search for aftershock sequences of M > 5.5 intermediate-depth earthquakes in two subducting slabs under Japan, and perform a statistical analysis to see if variations in aftershock productivity can be linked to slab properties. Our study regions are the older, colder and steeper Pacific Plate and the younger, warmer, shallower Philippine Sea Plate. We find that productive aftershock sequences behave similarly to those of crustal earthquakes and tend to appear mainly in the Pacific Plate where they are significantly correlated with along-strike variations in VP−VS ratio, suggesting a role for fluids in enabling intermediate-depth aftershock activity.